JPS6123383A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To cause stable single longitudinal mode oscillation and reduce the spectral width, by forming a diffraction lattice having a small coupling coefficient on a waveguide, and increasing the length of a resonator. CONSTITUTION:A diffraction lattice 5 having a small coupling coefficient K is provided between an active layer 2 and a trapping layer 3, and the resonator has a sufficiently large length L. The diffraction grating 5 may be provided between the active layer 2 and a substrate 1 rather than between the active layer 2 and the trapping layer 3. For instance, the whole of the waveguide is formed from the active layer 2, and the resonator length L is set so as to be 1,000mum or more, while the coupling coefficient of the diffraction lattice 5 is set so as to be 10cm<-1> or less.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a semiconductor laser that stably oscillates in a single longitudinal mode and has a narrow oscillation spectrum width.

[Prior art]

In conventional semiconductor lasers, the first
The method shown in the figure was used. In this figure, 1 is the substrate,
2 is an active layer, 3 is a confinement layer, and 4 is an end face formed by cleavage.

Next, the operation will be explained.

In this element, light is reflected by both end faces 4, and laser oscillation occurs between them. Oscillation spectrum width Δf1/2
is theoretically.

・・・・・・・・・(1) It is expressed as follows. Here, vg is the group velocity of light in the resonator,
hν is photon energy, g is threshold gain, and nsp is called a spontaneous emission coefficient, which takes an almost constant value. In addition, R is the power reflectance of the end face 4, Po is the output power, L is the cavity length, and α is the amount of change in the real part of the refractive index when the electron density changes slightly, which is the amount of change in the imaginary part. It can be considered as a constant.

As can be seen from this equation (1), if the resonator length is increased, the spectral width Δf I72 decreases in inverse proportion to it.

On the other hand, the longitudinal mode spacing Δλ (input is the wavelength) is Δin = 2L/
It is expressed as neff. Here, neff=n(1-person/n fistd
n/d) is the effective refractive index including wavelength dispersion.

Therefore, when the resonator length is increased, the longitudinal mode spacing Δin decreases, and while the gain of the medium is gradual with respect to temperature changes as shown in curve ■ in Figure 2, the threshold gain g is As shown in π, it changes greatly in response to changes in temperature,
This has the disadvantage that mode selectivity deteriorates and unstable phenomena such as mode hopping are more likely to occur.

[Summary of the invention]

This invention was made in order to eliminate the drawbacks of the conventional ones as described in "-", and by creating a diffraction grating with a weak coupling coefficient on the waveguide of a semiconductor laser and forming a long resonator, it becomes stable. The purpose is to achieve single longitudinal mode oscillation and narrow the spectral width.

Hereinafter, this invention will be explained with reference to the drawings.

[Embodiments of the invention]

FIG. 3 shows an embodiment of the present invention. In this figure, 1 is a substrate, 2 is an active layer, 3 is a confinement layer, and 4 is a confinement layer.
is an end face, a diffraction grating 5 having a weak coupling constant K is provided between the active layer 2 and the confinement layer 3, and
The resonator length is set to be sufficiently larger than the resonator length shown in FIG. In the embodiment shown in FIG. 3, the diffraction grating 5 is provided between the active layer 2 and the confinement layer 3, but it may also be provided between the active layer 2 and the substrate 1.

Next, the operation will be explained.

In the semiconductor laser shown in FIG. 3, the threshold condition is determined by the following equation.

(δ+jg)=iβcot(βe L) where, β”e = [(δ+jg)2−2) where i is an imaginary unit and δ is the deviation of the oscillation wave number from the Bragg wave number by 9g
is the gain, is a constant.

Figure 4 shows this result obtained by numerical calculation.

Here, the horizontal axis is the normalized resonator length KL, and the vertical axis is the normalized threshold gain gL. As can be seen from this figure, if the resonator length KL is greater than a certain level, the threshold gain of the first mode (model) will be lower than that of the other modes, mode2. Since the ratio is sufficiently small for mode 3, . . . , oscillation occurs in a stable single mode.

Further, although a theory regarding the spectral width of this semiconductor laser has not been clearly given, it can be inferred from the theory of conventional semiconductor lasers that the longer the resonator length becomes, the narrower the spectral width becomes.

However, even if it is increased beyond a certain level, most of the reflections occur near the center, and the effective cavity length Left
doesn't get bigger. Here, if the coupling coefficient K is made smaller, the reflection from the vicinity of the end face 4 will also contribute greatly, the effective resonator length L eff will increase, and the spectral width will also become narrower. L and Le
Figure 5 schematically shows the relationship between ff and
Figure 6(a).

(b) shows the state of reflection within the resonator depending on the magnitude of the coupling coefficient.

In the embodiment shown in FIG. 3, the entire waveguide is used as the active layer 2,
For example, the resonator length is l O00Jj,m or more, and the coupling coefficient of the diffraction grating is 10 cm' or less.

In the example shown in FIG. 3 above, the entire waveguide is used as the active layer 2 and a diffraction grating 5 is provided on the entire surface, but FIG. 7(a)
, (b), the active layer 2 and the Bragg reflection region of the waveguide 6 may be separated. Figure 7(a),
In the embodiment (b), the total length is set to be 1500 g or more, and the coupling efficiency of one diffraction grating 5 is set to 10 cm' or less. In this case as well, it is necessary to reduce the coupling coefficient K and increase the effective resonator length L eff.

Alternatively, as shown in FIG. 8, the entire waveguide may be made into an active layer 2, and a diffraction grating may be provided in which the coupling coefficient K is weak near the center and the coupling coefficient is strong near the ends.

In addition, in the second embodiment, DH (double heterojunction) is used in the vertical direction, BH (buried stripe) structure, C3P (channel substrate stripe) structure, and
It is also possible to confine light and carriers by using a TJS (lateral junction stripe) structure, an oxidized stripe structure, or the like.

〔Effect of the invention〕

As described above, in this invention, the waveguide is long and a diffraction grating with a small coupling coefficient is provided on a part or the whole of the waveguide, so that a narrow oscillation spectrum can be stably obtained.

[Brief explanation of drawings]

Figure 1 is a cross-sectional side view of a conventional semiconductor laser, Figure 2 is a diagram showing the relationship between the threshold gain and medium gain of a conventional semiconductor laser with a long cavity length, and Figure 3 is a diagram showing the relationship between the threshold gain and the gain of the medium in a conventional semiconductor laser with a long cavity length. A cross-sectional side view of one embodiment, FIG. 4 is a diagram showing the relationship between normalized threshold gain and normalized cavity length for each mode, and FIG. 5 is a diagram showing the relationship between normalized threshold gain and normalized cavity length for each mode. Diagram showing the relationship between length and effective resonator length, Figure 6 (a), (b)
FIG. 7(a) is a diagram showing the state of light reflection within the resonator when the coupling coefficient becomes large and small. (b) and FIG. 8 are side sectional views showing other embodiments of the present invention. In the figure, 1 is a substrate, 2 is an active layer, 3 is a confinement layer, 4 is a cleaved end face, 5 is a diffraction grating, and 6 is a waveguide. Note that the same reference numerals in the figures indicate the same or corresponding parts. Agent: Masuo Oiwa (2 others) Figure 5, Figure 6, Figure 7, Figure 8 Procedural amendment (voluntary) Showa year month/day 1, case description Patent application No. 145312/1989 2,
Title of the invention Semiconductor laser 3, Relationship with the case of the person making the amendment Hitoshi Katayama, representative of the patent applicant, Department 4, Agent 5, Detailed description of the invention in the specification to be amended and Drawing 6, Contents of the amendment (1) r(Hα2) in formula (1) on page 2, line 19 of the specification
” is corrected to “(1+α″)”. (2) Similarly, "constant" on page 5, line 9 is corrected to "coupling constant indicating the strength of the diffraction grating." (3) Correct Figure 3 as shown in the attached sheet. that's all

Claims (4)

[Claims] (1) In a semiconductor laser in which oscillation light generated in the active layer is extracted from a waveguide, the waveguide is made long in order to oscillate laser light in a stable single longitudinal mode and with a narrow oscillation spectrum width. Alternatively, a semiconductor laser is characterized in that a diffraction grating with a small coupling coefficient is provided throughout. (2) The cavity length is 1000 with the entire waveguide as the active region.
A semiconductor laser according to claim 1, characterized in that a diffraction grating having a diameter of .mu.m or more and a coupling efficiency of 10 cm^-^1 or less is provided over the entire waveguide. (3) A claim characterized in that the waveguide is divided into an active region and a black reflective region, and a diffraction grating with a coupling efficiency of 10 cm^-^1 or less is provided only in the black reflective region, and the total length is 1500 μm or more. The semiconductor laser according to item (1). (4) A semiconductor laser according to claim (1), characterized in that the entire waveguide is used as an active region, and a diffraction grating is provided in which the coupling efficiency is increased near the center.