JPH04320081A - Semiconductor light source - Google Patents

Abstract

PURPOSE:To provide a distribution reflective type semiconductor light source which oscillates in a single vertical mode and whose spectrum line width is narrow even when the wavelength is variable. CONSTITUTION:An optical fiber 4 having specified length is connected to the light-emitting plain of a wavelength-variable distribution reflective type semiconductor laser 1a. A part of the light-emitted from the distribution reflective laser 1a and entering the optical fiber 4 becomes backward scattering light due to the structural unevenness that the optical fiber 4 itself possesses and automatically becomes a return light to the distribution reflective type semiconductor laser 1a, making the spectrum line width narrow.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention oscillates in a single longitudinal mode,
The present invention also relates to a distributed reflection type semiconductor light source that has a narrow spectral linewidth even when the wavelength is variable.

0002

[Prior art] In a Fabry-Perot type semiconductor laser,
Longitudinal modes exist at certain intervals that satisfy the phase condition. Since there is no loss difference between these longitudinal modes, even if single-mode operation is obtained during DC operation, the gain distribution will fluctuate drastically when high-speed direct modulation is performed due to slight gain fluctuations, so multi-mode operation mode hopping occurs. Longitudinal mode control is a method of considering the structure of a semiconductor laser so that a stable single longitudinal mode can be achieved in response to this problem. One way to solve this problem is to create a loss difference depending on the wavelength in the resonator. Examples of this are distributed feedback lasers (hereinafter also referred to as DFB lasers) and distributed reflection semiconductor lasers (hereinafter also referred to as DFB lasers). There are also lasers (also called DBR lasers), lasers with external diffraction gratings, etc. Among them, DBR lasers are said to be less susceptible to spatial hole burning that causes carrier non-uniformity and have a structure that allows stable single longitudinal mode oscillation to be easily obtained. Regarding the wavelength tunable effect, DBR lasers change the refractive index of the guide layer using the plasma effect caused by current injection, and DF lasers change the refractive index by thermal effects.
Compared to the B laser, it has the advantage of being able to perform wavelength tuning quickly and stably.

[0003] The ability to vary the wavelength in this way is
One of the characteristics of the R laser is that the spectral linewidth deteriorates significantly when the wavelength is varied. The cause of this is thought to be the poor electrical insulation between the light emitting region and the wavelength control region, and the setting of the pn junction.However, the fundamental cause is that the guide for generating the plasma effect is insufficient. One example is that the refractive index of the light emitting region is affected by carrier injection into the layer.

A structure using an external resonator is known as a general method for achieving a single longitudinal mode and narrowing the spectral linewidth. In this structure, the light emitted from the end face of the light emitting element is reflected by an external mirror, a diffraction grating, a fiber end face with a high reflection film formed on one end face, etc., and then returned to the light emitting element. Achieved widening. The reason why the spectral linewidth becomes narrow is that the distance between the resonators, that is, between the element and the reflecting surface becomes longer and the resonance amplitude becomes smaller.

[0005] Another method of narrowing the spectral line width is a configuration using a quantum well structure. In addition to the multi-quantum well structure, it is said that the spectral linewidth can be narrowed by making the device itself a long resonator. For example, the Proceedings of the 1991 Spring National Conference of the Institute of Electronics, Information and Communication Engineers (C
148, Matsui et al.), the minimum spectral linewidth is narrowed to 85 KHz in a multi-quantum well structure DBR laser with an element length of 2.2 mm.

[0006]

[Problems to be Solved by the Invention] Next, problems faced by these conventional techniques will be described. First, the biggest problem with DBR lasers is that the spectral line width widens when the wavelength is varied. Various methods have been proposed to solve this problem, but each method has its own problems.

In a structure using an external resonator to narrow the spectral linewidth, the feedback light to the laser is spatially coupled between the end facet of the laser element and the external reflector or grating, so that heat is generated. The problem is that the phase of the return light at the laser end face tends to deviate from the optimum value, and the oscillation wavelength, frequency, and spectral line width tend to fluctuate, as well as the mechanical integrity of the device. There are problems in that it is difficult to maintain accuracy and the mechanism for maintaining accuracy is complicated. Further, in the configuration of an external resonator using a fiber with a high reflectance film formed on one end face as an external reflecting mirror, there is a problem that it is difficult to monitor the light output from the high reflection film side. More fundamentally, a DBR laser has a problem in that it is impossible to attach an external resonator because light is emitted from only one end facet.

[0008] Lasers with a multi-quantum well structure, in which the structure of the element itself has been improved and the element length has been lengthened, can achieve narrower spectral linewidths than those with conventional bulk structures, but the manufacturing yield is There are difficulties in maintaining it. Furthermore, coherent communication systems require narrower linewidths, and even with structures using quantum wells, it is still difficult to narrow the spectral linewidth to a level of 10 KHz or less.

[0009]

[Means for Solving the Problems] Here, means for solving the above problems will be described. The present invention provides an optical fiber 4 having a predetermined length on the emission surface of the distributed reflection semiconductor laser 1 in order to narrow the spectral line width even when the wavelength of the distributed reflection semiconductor laser 1 is changed. Connecting. A part of the light incident on the optical fiber 4 from the distributed reflection type semiconductor laser 1 becomes backscattered light due to the structural non-uniformity of the optical fiber 4 itself, and is automatically transmitted to the distributed reflection type semiconductor laser 1. It becomes feedback light and makes it possible to narrow the spectral line width.

0010

[Operation] Light emitted from the element end face of the distributed reflection type semiconductor laser 1 passes through an optical coupling part such as a lens interposed between the element and the optical fiber 4, and enters the optical fiber 4. Generally, the optical fiber 4 has slight non-uniformity in the core diameter and refractive index that serve as a waveguide, resulting in a non-uniform refractive index distribution along the light propagation direction. . In other words, this refractive index distribution becomes a distribution constant minute reflection source, and a part of the light incident on the optical fiber 4 becomes backscattered light (hereinafter referred to as
(also referred to as back scatter) and becomes a minute amount of feedback light to the DBR laser 1. The intensity of this backscattered light is about -40 dB when the optical fiber length is about 1 km, and it has sufficient light intensity required for linewidth narrowing, and the external cavity length is effectively long. Therefore, it is possible to easily narrow the spectral line width. In other words, the optical fiber 4 serves as a light waveguide/propagator as well as a reflecting mirror in a self-aligned manner, and a portion of the incident light becomes backscattered light and returns to the DBR laser 1. The line width is narrowed.

[0011]

Embodiments Examples of the present invention will be described below based on the drawings. FIG. 1 shows a first embodiment. As the DBR laser 1, a three-electrode DBR laser 1a is used. The light emitted from the three-electrode DBR laser 1a is incident on the spectral line width narrowing fiber 4a, which is a single mode optical fiber 4, via the optical coupling section 3. Generally, a spherical lens, selfoc lens, etc. is used for the optical coupling part 3, but here, one tip of the fiber is processed into a spherical shape so as to have a lens effect, and the optical coupling part 3 and a fiber for spectral line width narrowing are used. 4a is integrated. A part of the incident light becomes backscattered light and enters the three-electrode DBR laser 1a.
, and the spectral linewidth is narrowed. Here, the fiber 4a for spectral line width narrowing is a low-loss waveguide and propagation body for light, and at the same time, it also serves as a source of backscattered light as feedback light to the laser necessary for narrowing the spectral line width. In order to narrow the spectral line width to about 10 KHz, a length of about 1 km is required. The other end of the spectral line width narrowing fiber 4a is coupled to the input side of an optical isolator 8, and the output side of 8 is coupled to a single mode optical fiber 9 having an optical connector 5 for light extraction. The optical isolator 8 has a reverse insertion loss of about 60 dB or more, and the reflection at the coupling part between the optical isolator 8 and the spectral line width narrowing fiber 4 is about -58 dB.
dB or less, and it is considered that the feedback light to the three-electrode DBR laser 1a is essentially backscattered light generated in the spectral line width narrowing fiber 4a with an intensity of -40 dB.

FIG. 2 shows the relationship between the length of the spectral linewidth narrowing fiber 4a and the spectral linewidth obtained in the configuration of the first embodiment. The spectral linewidth was measured by a delay self-heterodyne method using a 100 km single mode optical fiber for delay on the light emitted from the single mode optical fiber 9 for communication. The laser diode (hereinafter also referred to as LD) used in FIG. 2 is the three-electrode DBR laser 1a having an active region, a phase control region, and a distributed reflection region used in the first embodiment. By controlling the injection current, the oscillation wavelength can be changed from 1.549 μm to 1.549 μm.
It has the characteristic that the wavelength can be varied with phase continuity up to 547 μm. When a current is injected only into the active region and the optical output is controlled to 3 mW, the spectral linewidth of this LD alone is approximately 3 mW.
It was 7MHz. However, when the wavelength was shifted to the shorter wavelength side by about 1.8 nm while maintaining the same optical output, the spectral linewidth expanded to about 100 MHz or more. In FIG. 2, the vertical axis represents the spectral linewidth, and the horizontal axis represents the length of the single mode optical fiber. In Figure 2, the ○ symbol (
LD1) is when current is injected only into the active region of the above LD, □ symbol (LD2) is when a current source is also connected to the other two electrodes, and △ symbol (LD3) is when the wavelength is approximately shifted to the short wavelength side. This is a plot of measured values when shifted by 1.8 nm. Note that the optical output was conducted under a constant condition of 3 mW.

From FIG. 2, it can be seen that the spectral linewidth is rapidly reduced by using the spectral linewidth narrowing fiber 4a in the LD, and that the spectral linewidth narrowing fiber 4a
It can be seen that there is a tendency for the reduction effect to saturate at about 1 km, and that a constriction effect can be obtained even when the wavelength is shifted. The spectral line width of other lasers having the same structure as this LD was 29 MHz when the LD was used alone, but when it was coupled for 1 km as the spectral line width narrowing fiber 4a, the spectral line width decreased to 6.2 KHz. Also, the wavelength of LD1 is -
When shifted by 1.7 nm, the spectral line width is 6.0 K when the fiber 4a for spectral line width narrowing is coupled for 1 km.
Hz was obtained. As a result of the above results, in this system using backscattered light, a narrow spectral linewidth can be easily obtained, and the effect of narrowing the spectral linewidth by about 1/5000 can be obtained, and it is also effective after a wavelength shift. I understand that.

[0014] Although it has been demonstrated that this method of coupling an optical fiber to a DBR laser can narrow the spectral line width even when the wavelength is changed,
It is clear that FB lasers also have this effect. This DFB laser will be described below using FIGS. 3 and 4. FIG. 3 shows an example in which a DFB laser 1b is used and backscattered light as feedback light is introduced from the front side of the laser 1. In this example, the DFB laser has a wavelength of 1.3 μm.
A DFB laser with an asymmetric reflectance structure (front reflectance of 1%, rear reflectance of 30%) is used. DFB
On the front side of the laser 1b, a single longitudinal mode is promoted,
Furthermore, an antireflection film 2 made of silicon oxide is provided for the purpose of increasing light extraction efficiency. The light emitted from the DFB laser 1b is incident on a spectral line width narrowing fiber 4a which has a spherical tip and is integrally coupled with an optical coupling part 3 having a lens function. The length of the spectral line width narrowing fiber 4a is required to be about 1 km in order to achieve a spectral line width of 10 KHz. A part of the incident light becomes backscattered light and returns to the DFB laser 1b, thereby narrowing the spectral line width. Light with a narrowed spectral linewidth is emitted from both the front and rear surfaces of the DFB laser 1b. Although omitted in the second embodiment, it is clear that the light from the rear side of the laser 1 can also be used for monitoring the optical output. The optical isolator 8 has a reverse insertion loss of about 60 dBm or more,
Return light from ahead of the optical isolator 8 is prevented from returning to the laser 1. The spectral linewidth was determined by measuring the emitted light from the single mode optical fiber 9 using the delayed self-heterodyne method described above, and found that the spectral linewidth of the LD1 alone was 36 MHz, but the spectral linewidth of the spectral linewidth narrowing fiber 4a was 1 km. When coupled, it was reduced to 50KHz.

FIG. 4 shows an example in which a DFB laser 1b is used and backscattered light as feedback light is introduced from the rear side of the laser 1. In this example, the DFB laser has a wavelength of 1.3μ.
A DFB laser 1b with an asymmetric reflectance structure (front reflectance of 1%, rear reflectance of 30%) that oscillates at m is used. An antireflection film 2 made of silicon oxide is provided on the front side of the laser for the purpose of promoting single longitudinal mode and increasing light extraction efficiency. The light emitted from the rear surface side of the laser 1 is input into the spectral line width narrowing fiber 4a via the optical coupling part 3 whose tip is processed into a spherical shape and has a lens function. The fiber 4a for spectral line width narrowing requires a length of about 1 km for narrowing the spectral line width. A part of the incident light becomes backscattered light and is emitted by laser 1.
Returning to the previous step, the spectral linewidth is narrowed. Light with a narrowed spectral linewidth is emitted from both the front and rear surfaces of the laser 1. The light emitted from the front surface of the DFB laser 1b is coupled to a single mode optical fiber 9 through a ball lens 6, an optical isolator 8, and a selfoc lens 7 for focusing. The spectral linewidth was measured using the delayed self-heterodyne method described above for the light emitted from the single mode optical fiber 9, and the spectral linewidth of the LD itself was 25 MHz, but the spectral linewidth of the spectral line width narrowing fiber 4a was 1K.
When m-coupled, it decreased to 50KHz.

Furthermore, the method of coupling an optical fiber to a DBR laser, which can narrow the spectral line width even when the wavelength is changed, is also effective in the configuration of an external resonator. FIG. 5 shows an example in which backscattered light is introduced from the front side of a laser having an external cavity configuration to narrow the linewidth. In this embodiment, a Fabry-Perot laser 1c on which an antireflection film 2 is formed is used as the laser 1. The light emitted from the rear surface of the Fabry-Perot laser 1c, that is, from the side of the anti-reflection film 2, passes through the lens 7, focuses on the diffraction grating 10, selects its wavelength, and returns the diffracted light, which has a single wavelength, to the Fabry-Perot laser 1c via the Selfoc lens 7. By returning to the Perot type laser 1c, light in a single longitudinal mode is emitted from the front of the laser. The spectral line width of the light emitted from the front side of the Fabry-Perot laser 1c has already been narrowed to about 200 to 500 KHz at this stage, but in order to further narrow the spectral line width, it is passed through the Selfoc lens 7 to narrow the spectral line width. The light is input to the constricting fiber 4a. For example, if the spectral linewidth is 1
In order to narrow the spectrum to about 0 KHz, the length of the fiber 4a for narrowing the spectral line width is required to be about 1 km. A part of the incident light becomes backscattered light and enters the Fabry-Perot laser 1.
Returning to step c, the spectral linewidth is narrowed. The light having the narrowed spectral linewidth is emitted from the optical isolator 8 coupled to the spectral linewidth narrowing fiber 4a and the optical connector 5 coupled to the tip of the single mode optical fiber 9 for communication. .

[0017]

Effects of the Invention The distributed reflection type semiconductor laser according to the present invention can narrow the spectral linewidth of the emitted light to about 10 kHz even when the wavelength is variable by connecting the fiber for spectral linewidth narrowing to the emission end facet. It became possible to do so. As a result of the narrower spectral linewidth, it becomes possible to multiplex optical communications and transmit even larger amounts of information. Further, since the structure is such that an optical fiber of a predetermined length is coupled to the element of the distributed reflection type semiconductor laser, it is very easy to manufacture. In a distributed reflection type semiconductor laser, light is emitted only from one side, but with the configuration of the present invention, the spectral line width can be narrowed without using a reflecting mirror, so the emitted light can be directly emitted from the narrowed side. and can be used for communication.

[0018]

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing fiber length versus spectral linewidth characteristics in the first embodiment of the present invention.

FIG. 3 is a diagram showing a second embodiment of the present invention.

FIG. 4 is a diagram showing a third embodiment of the present invention.

FIG. 5 is a diagram showing a fourth embodiment of the present invention.

[Brief explanation of symbols]

1 DBR laser. 1a 3-electrode DBR laser. 1b DFB laser. 1c Fabry-Perot laser. 2. Anti-reflection film. 3. Optical coupling part. 4. Optical fiber. 4a Fiber for spectral line width narrowing. 5. Optical connector. 6. Ball lens. 7 Selfoc lens for condensing light. 8. Optical isolator. 9 Single mode optical fiber. 10 Diffraction grating.

Claims (1)

[Claims] Claim 1: Tunable wavelength distributed reflection semiconductor laser (1)
and an optical fiber (4) having a predetermined length, one end of which is coupled to the emission surface of the semiconductor laser, and a wavelength tunable unit having a narrow spectral linewidth extending from the other end of the optical fiber. A semiconductor light source that emits light in one longitudinal mode.