JPS63104492A - Wavelength control distributed bragg-reflector semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a title laser, which is capable of controlling precisely the wavelength, is stably actuated in a small size and moreover can be easily manufactured, by a method wherein the effective refractive indexes of a distributed Bragg reflector (DBR) part and a filter part are changed by injecting carriers in these parts and are corrected by just adding a simple feedback circuit. CONSTITUTION:The transmittance of light of a specified wavelength incident through a DBR part 200 is changed by injecting carriers in a filter part 300. This matter adversely can monitor the fluctuation of an oscillation wavelength by a method wherein the transmission characteristic of a filter is electrically controlled and fixed in a certain state and the amount of transmitted light is received. Accordingly, by controlling the injected current of the DBR part 200 in such a way as to keep the light-receiving level at a constant value using an electrical feedback circuit, the oscillation wavelength can be kept at a constant value. The stability of the wavelength can be stabilized in 0.3 nm or thereabouts, and moreover the control extent of the wavelength also can be controlled over several nm or more of the roughly same degree as the wavelength variable extent of a laser.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a wavelength-controlled distributed Bragg reflection type semiconductor laser used as a light source in optical communications and the like.

(Prior Art) Future high-density wavelength division multiplexing optical transmission systems will require a semiconductor laser light source whose wavelength is precisely controlled. Therefore, for K, it is possible to use a single-axis mode laser such as a distributed feedback laser or a distributed Bragg reflection laser (hereinafter referred to as a DBR laser).

However, controlling the oscillation wavelength of these lasers to a desired value becomes more difficult as the multiplexed wavelength interval becomes narrower. This is because variations in active layer thickness etc. occur during the manufacturing stage of the laser.Currently, the limit is about 5 nm spacing in the 1.3 μm band. Therefore, in order to perform high-density multiplexing with an interval of K1 nm or less, the light source is required to have (1) a wavelength variable function and (2) a function to maintain the wavelength at a desired value. ■
Several prototype examples of wavelength tunable lasers have already been reported. In particular, a DBR laser having a wavelength tunable function is excellent in that the wavelength can be electrically changed over a relatively wide wavelength range and stable single-axis mode operation can be performed. Regarding this, for example, Electronics Letters magazine (Y).

Tohmori et al+ Electro
n, Lett.

22 (1986) 138).

Regarding the function (2) of keeping the wavelength at a desired value, there is currently no laser that has such a function added to the laser itself.

In order to keep the wavelength at a certain value, it is necessary to monitor the wavelength of the tunable laser and apply feedback to the laser so that this wavelength remains constant. This K requires some kind of wavelength selector, and conventionally optical equipment such as a Fapley-Bello interferometer or a diffraction grating has been used.

The optical output of the desired wavelength selected by these wavelength selectors is monitored by a receiver, and wavelength control is performed by applying electrical feedback to the wavelength control section of the tunable laser so that this output remains constant. It was done.

(Problems to be Solved by the Invention) As described above, in order to accurately control the wavelength of a laser, the wavelength selection function as well as the wavelength variable function of the laser are important. However, conventionally, using an optical device such as a Fapley-Bello interferometer as a wavelength selector has been problematic in terms of size, ease of manufacture, and cost. Furthermore, it was insufficient in terms of long-term stability.

SUMMARY OF THE INVENTION An object of the present invention is to improve the above-mentioned problems and provide a wavelength-controlled laser that can accurately control wavelength, is compact, operates stably, and is easy to manufacture.

(Means for solving the problem) The present invention includes an active region and a distributed Bragg reflector (hereinafter referred to as D) connected to the active region and transparent to the oscillation wavelength.
BR) part and a waveguide grating filter part connected to the DBR part and having the same composition as the DBR part on one semiconductor substrate, and the effective refraction of the DBR part and the filter part is The above-mentioned problems are solved by a wavelength-controlled DBR laser characterized in that the DBR section and the filter section are each provided with electrodes that control the rate independently of each other.

(Function) FIG. 1 is a cross-sectional view in the axial direction of a laser resonator, showing the basic structure of the present invention. The laser of the present invention has an active region io
2, a DBR section 200, and a waveguide grating filter section 300, all of which are formed and integrated on a semiconductor substrate 500. The wavelength variable mechanism of this laser has already been explained in the above-mentioned literature, but will be briefly explained below. Of the light emitted by electrical injection into the active region 100, only light of a specific wavelength is strongly reflected by the DBR section 200, and laser oscillation occurs at a vague wavelength. This wavelength is mainly determined by the effective refractive index of the DBR section 200 and the pitch of the diffraction grating, that is, the Bragg wavelength. Therefore, by electrically changing the effective refractive index of the DBR section 200, the Bragg wavelength can be changed and the oscillation wavelength of the laser can be changed. Methods for electrically changing the effective refractive index include a method of injecting carriers into the light guide eyebrow 501 of the DBR section 200 and a method of using an electro-optic effect. In the method of injecting carriers, the continuously variable wavelength range is 7 and 1.5 μm. nm, several nm or more is possible in a continuous manner. This situation is schematically shown in FIG. 2(a). Here ID is DBR section 2
00, and the current in the active region 100 is kept constant at this time. Although not directly mentioned in the present invention, the discontinuous wavelength change as shown in the figure can be made continuous by adding a phase control section to such wavelength tunable I) BR laser. Yes, this point is already known. The wavelength control described below can be similarly applied to such a continuously variable wavelength laser.

Next, the wavelength control mechanism will be described. A filter unit 300 is used as a wavelength selector for controlling the wavelength of a wavelength tunable laser to a specific wavelength as shown in FIG. 2(a). Since this filter section 300 has the same optical guide layer 501 and diffraction grating as the DBR section 200, the reflection characteristics are as follows.
It exhibits almost the same characteristics as the DBR section 200. That is, by electrically changing the effective refractive index of the filter section 300, reflection or transmission characteristics can be changed. This situation is shown in FIG. 2(b).

That is, by injecting carriers into the filter section 300, it is possible to change the transmittance of light of a specific wavelength incident from the DBR section 200 (in FIG. 2(b), the current injected into the filter section 300 is designated as IF). ). Conversely, this can be achieved by electrically controlling the transmission characteristics of the filter, fixing it in a certain state, and receiving the amount of transmitted light.
This shows that it is possible to monitor fluctuations in the oscillation wavelength. Therefore, the DBR unit 20 maintains the received light level at a constant value.
0 injection current using an electrical feedback circuit,
By controlling it, the oscillation wavelength can be kept at a constant value. The wavelength stability varies depending on the characteristics of the filter, but it can be stabilized to about 0.3 nm, and the wavelength control range can also be controlled over several nm or more, which is approximately the same as the wavelength variable range of the laser.

(Example) FIG. 3 is a perspective view showing an embodiment of the present invention.

Basically, the structure is similar to that shown in Fig. 1, and the active region is 100°D.
Consisting of a BR section 200 and a filter section 300,
It is integrated on the same InP substrate.

Active region 100, DBR section 200, filter section 300
The length is 150μm, 500μm and 1500μm respectively.
It is. The manufacturing procedure will be briefly described below. First, n-type In
DBR section 200 and filter section 300 on P substrate 500
A diffraction grating with a period of 240 nm is formed. Next, by the first LPE, the n-type InGaAsP light guide layer 50 is
1. An InP buffer layer 502, an InGaAsP active layer 503, and a P-type InP cladding layer 504 are sequentially grown. Next, a DBR section 200 with a diffraction grating and a filter section 300
The cladding layer 504 and active layer 503 are selectively removed. The light guide layer 501 is left as is. In the second LPE growth, a P-type InP cladding layer 505 is formed over the entire structure. Next, in order to form a buried structure, mesa etching is performed, and then buried growth is performed by a third LPE growth. In this embodiment, a double channel planar embedded type was used, but other striped structures can also have essentially the same effect. Next, active regions 100 on the substrate side and the growth side
, electrodes are formed on the DBR section 200 and the filter section 300. Electrical isolation of each part was achieved by etching a part of the grown layer and providing grooves. The light guide layer 501 is transparent to the laser oscillation wavelength. Further, since a p-n junction is formed, by injecting carriers into the DBR section 200 and the filter section 300, the effective refractive index of this section can be changed. An example of the characteristics of the device manufactured as shown in Part B is shown below. Threshold 16 m
A.

External differential quantum efficiency from the front 25%, oscillation wavelength 1.55
It was μm. When current is injected into the DBR section, the variable wavelength range is Q, 9 nm continuously, and 4.5 nm discontinuously.
It was m. The optical output characteristics from the filter section 300, that is, the filter characteristics, were measured by keeping the current injected into the filter section 300 constant and changing the oscillation wavelength by υ depending on the current injected into the DBR section 200. The maximum light output from the filter section 300 was 8% of the light output from the front surface, and the minimum was 1%. The full width at half maximum of the transmission spectrum is 0.6n, m
Met. Simple electrical feedback as described in the (effect) section was applied to this element to control the current of the DBR section 200. When the current injected into the active region 100 is increased to cause wavelength fluctuations due to heat generation, in the absence of feedback, a wavelength shift of about Inm occurs, whereas in the case of feedback, the wavelength shift is about 0.3 nm.
We were able to suppress the wavelength shift to .

In the above-described embodiment, the effective refractive index was changed by carrier injection, but an electro-optic effect may also be used. In this case, higher performance can be expected by further forming the optical guide layer 501 into a quantum well structure. Further, by simultaneously integrating nine light receiving sections for monitoring the optical output from the filter section 300, it is possible to further downsize the feedback system.

(Effects of the Invention) As described above, according to the present invention, a small and stable wavelength-controlled semiconductor laser can be realized. That is, fluctuations in the oscillation wavelength due to heat generation, deterioration, etc. in the active region can be corrected by simply adding a simple feedback circuit. Such a laser is effective as a light source for future high-density wavelength division multiplexed optical transmission.

[Brief explanation of the drawing]

Figure 1 is a sectional view showing the basic structure of the present invention, Figure 2 (
a) is a characteristic diagram showing the fluctuation of the oscillation wavelength due to injection current control of the DBR section in the structure of Fig.
FIG. 3 is a characteristic diagram showing the relationship between the transmittance of the filter section and the oscillation wavelength in the structure shown in the figure, and FIG. 3 is a perspective view showing an embodiment of the present invention. In the figure, 100 is an active region, 200 is a DBR section, 300 is a filter section, 500 is a substrate, 501 is a light guide layer, 5
02 is a buffer layer, 503 is an active layer, 504, 505
is the cladding layer. Agent Patent Attorney Shinsuke Honsho 3S SE L 乍東上八 〇BR9filter 杏P 1 0 ji master, LIo eJA disease &
(a) (b)

Claims (1)

[Claims] an active region, a distributed Bragg reflector section connected to the active region and transparent to the oscillation wavelength, and a waveguide grating filter connected to the reflector section and having the same composition as the reflector section. on one semiconductor substrate, and each of the reflector section and the filter section is provided with an electrode that controls the effective refractive index of the reflector section and the filter section independently of each other. A wavelength-controlled distributed Bragg reflection type semiconductor laser.