JPH11150324A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser of a structure, wherein temperature dependence of oscillation wavelength of the laser is reduced and the oscillation wavelength is stable to a temperature change. SOLUTION: Distributed reflectors having different temperature dependences of effective refractive index are formed on both sides on the front and rear of an active region 2 of a distributed reflection type semiconductor laser, and sample diffraction gratings 6 and 7 are respectively formed on these distributed reflectors. The ratio of the temperature gradient to a change in the refractive indexes of optical waveguides 4 and 5 on the front and rear of this active region 2 to the sample period of the diffraction gratings 6 and 7 contrives so as to become constant to constitute a vernier structure using the distributed reflectors formed on the front and rear of the above region 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly, to stabilizing an oscillation wavelength of a semiconductor laser against a change in temperature.

[0002]

2. Description of the Related Art Distributed feedback lasers (DFBs) which have been put to practical use as semiconductor lasers for high-speed optical communication.
edback laser diode) and distributed reflection type (DBR: distribute)
In a semiconductor laser having a Bragg diffraction grating such as a ted Bragg reflector, the oscillation wavelength is determined by the equivalent refractive index of the waveguide and the period of the diffraction grating. However, since the refractive index of the semiconductor constituting the laser has temperature dependence, the oscillation wavelength of the laser changes to a longer wavelength side at about 0.1 nm / ° C. as the element temperature rises. For this reason, wavelength division multiplexing transmission (WDM: wavelength division mu) in which wavelength control accuracy is particularly important.
In a ltiplexing system or the like, temperature control by a Peltier element of a semiconductor laser is indispensable, and therefore, power consumption is increased and the device is enlarged.

As a method of reducing the temperature dependence of the oscillation wavelength of such a laser, for example, Japanese Patent Application Laid-Open No. 60-558 discloses a method.
As shown in FIG. 8, there is known a method of using a material having a negative temperature dependency, in which the refractive index decreases with a rise in temperature, or a material in which a part of the laser resonator is shortened by the rise in temperature. Also, for example, as shown in JP-A-9-92924, a method is known in which a dielectric or the like having a negative refractive index with respect to temperature is used for a reflector. In these methods, a material having a negative temperature dependence of the refractive index is used, thereby compensating for the positive temperature dependence of the refractive index of the semiconductor material constituting the laser and stabilizing the wavelength.

[0004]

However, the material having the negative temperature dependence of the refractive index is an organic material such as polystyrene or PMMA, or a dielectric such as lithium fluoride. Is not always easy to apply, and sufficient results are not always obtained with respect to reliability and long-term stability of laser characteristics as compared with ordinary semiconductor lasers.

An object of the present invention is to provide a laser having a small temperature dependence of an oscillation wavelength by using a normal semiconductor laser manufacturing process.

[0006]

As a result of various studies to achieve the above object, the following invention has been achieved by combining a plurality of semiconductor waveguides having different temperature dependences of the refractive index. 1. A semiconductor laser having a Bragg reflector, comprising: a semiconductor laser having at least two types of optical waveguides having different temperature dependences of an equivalent refractive index and a diffraction grating structure provided in the optical waveguide and reflecting a plurality of wavelengths. laser. 2. 2. The semiconductor laser according to the above item 1, wherein the diffraction grating structure provided in the optical waveguide is a sample diffraction grating. 3. In the optical waveguide forming the plurality of sample diffraction gratings, the product of the equivalent refractive index of the optical waveguide and the diffraction grating period at an average temperature at least when the element temperature is changed is constant, and the change in the refractive index of the optical waveguide is constant. 3. The semiconductor laser according to the above 1 or 2, wherein the inverse ratio between the temperature gradient and the sample period of the sample diffraction grating is constant. 4. 4. A distributed reflection type having at least an active region, a passive region having a temperature dependency of an equivalent refractive index different on both sides of the active region in a resonator direction, and a sample diffraction grating in the passive region. Semiconductor laser. 5. 4. The semiconductor laser according to the item 3, wherein the semiconductor laser has at least an active region having the sample diffraction grating and a passive region having the sample diffraction grating. 6. 6. The semiconductor laser according to the above 4 or 5, further comprising a heating mechanism by current injection for controlling the temperature of the optical waveguide.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the principle of the present invention will be described with reference to FIG. FIG. 2 is a diagram for explaining the reason why the wavelength of the distributed reflection laser shown in FIG. 1 described later is stabilized. (A) to (d) in the figure show the reflection spectrum 2 at the front side of the device when the device temperature is 0 ° C., 20 ° C., 50 ° C. and 100 ° C.
2, and the reflection spectrum 23 on the rear surface side of the element is schematically shown with the horizontal axis as the wavelength. As shown in FIG. 2, the reflection spectrum interval of the front-side reflection spectrum 22 is narrower than the rear-side reflection spectrum 23 of the element, and the reflection spectra before and after the element constitute a vernier structure. The wavelength gradient 24 on the front side of the element is smaller than the temperature gradient 25 on the rear side of the element. A laser having a distributed reflector having such a structure oscillates at a wavelength at which both front and rear reflection spectra match. Therefore, the wavelength interval of the reflection spectrum hardly changes even when the temperature changes, so that the oscillation wavelength 21 hardly changes even when the element temperature changes.
That is, in the optical waveguide forming the two sample diffraction gratings, the average temperature (50
(° C), the center wavelengths of the reflection spectra match (the product of the equivalent refractive index of the optical waveguide and the diffraction grating period matches)
The structure has a structure in which the temperature gradient of the change in the reflection spectrum (temperature gradient of the change in the refractive index of the optical waveguide) and the ratio of the wavelength interval of the reflection spectrum (the inverse ratio of the sample period of the sample diffraction grating) are constant. Is compensated for the change in the wavelength with respect to.

According to the present invention, the temperature dependence of the refractive index differs depending on the composition due to the temperature dependence of the band gap energy and the wavelength dispersion characteristics of the refractive index.
This is based on the fact that the temperature dependence of the refractive index of the passive region is larger than that of the active region in which the threshold carrier density increases as the temperature increases and the refractive index decreases. Further, the present invention utilizes the fact that the refractive index changes almost linearly with a temperature change at a temperature at which a laser is usually used.

[0009]

Next, an embodiment of the present invention will be described with reference to the drawings.

Embodiment 1 FIG. 1 is a cross-sectional structure of a distributed reflection laser showing the configuration of a first embodiment of the present invention. As shown in FIG. 1, the peak of the emission wavelength is 1. on the n-InP substrate 1 by the MOVPE method.
A multiple quantum well (MQW) active region 2 having a thickness of 55 μm and a p-InP cladding layer 3 are grown. Next, active area 2
Is etched so as to have a length of 300 μm, and the optical waveguides 4 and 5 are provided on both front and rear sides of the active region 2 in the resonator direction.
Is grown by a selective MOVPE method with a waveguide sandwiched between SiO ₂ masks having different coverages. The compositions of the optical waveguides 4 and 5 are such that the forbidden band width corresponds to 1.20 μm in wavelength (InGaAsP 1.20 μm composition) on the front side (light output side) of the element, and the forbidden band width on the rear side of the element. It is manufactured so as to have a composition corresponding to a wavelength of 1.25 μm (InGaAsP 1.25 μm composition).

Next, an electron beam exposure method is applied to the front and rear optical waveguides to form a 660 μm optical waveguide on the front side of the device.
A sample diffraction grating 6 consisting of a diffraction grating having a period of 0.2400 μm is formed in a region of 20 μm at intervals of 0 μm, and a sample diffraction grating comprising a diffraction grating having a period of 0.2405 μm is formed in a region of 20 μm at intervals of 200 μm in a 600 μm optical waveguide on the rear surface side. A lattice 7 is produced.

On this optical waveguide, the IV
After growing the nP cladding layer 8, an optical waveguide having a stripe structure is formed according to a normal laser fabrication process,
The current block layer is regrown using the MOVPE method. Further, anti-reflection coatings 12 and 13 are applied to the n-side electrode 9, the p-side electrode 10, and both front and rear end faces of the element, thereby producing a laser.

The laser manufactured in this manner has a reflection temperature of 2
At 0 ° C., the front side of the device is spaced at 1.65 nm centered on a wavelength of 1.5477 μm, and the rear side of the device is at a wavelength of 1.5474
The interval is 1.80 nm centering on μm. Therefore, at 20 ° C., the laser oscillates at 1.5510 μm in which the reflection spectra before and after forming the vernier structure coincide.

The temperature dependence of each distributed reflector is 0.11 nm / ° C. on the front side of the element and 0.12 nm / ° C. on the rear side of the element.
nm / ° C., and when the element temperature rises by 7.5 ° C., the reflection spectrum changes to the longer wavelength side of 0.825 nm and 0.90 nm, respectively. Therefore, also in this case, since the reflection spectrum is equal at 1.5510 μm, the oscillation mode changes to the adjacent mode with respect to the temperature change, thereby realizing a laser having a small change in the oscillation wavelength. The device has such oscillation characteristics when the element temperature is between 0 ° C. and 100 ° C., and in this temperature range, the distribution of the oscillation wavelength is stabilized to about 1 nm or less.

In this embodiment, etching and regrowth are performed to obtain I
Although the nGaAsP optical waveguide was manufactured, it is also possible to form the optical waveguide region simultaneously with the formation of the active region using the selective MOVPE method. In this embodiment, the electrode is not formed in the diffraction grating forming region. However, the electrode may be formed by using the InP cladding layer 8 as a p-InP cladding layer. In this case, the electrode is formed in the diffraction grating forming region. By injecting the current, the oscillation wavelength can be finely adjusted. Further, in this embodiment, the sample diffraction grating is formed in the optical waveguide. However, it is sufficient that a reflection spectrum reflecting a plurality of wavelengths is formed, and such a reflection spectrum is formed by a phase shift diffraction grating and a periodic modulation diffraction grating. Is similarly possible.

Embodiment 2 FIG. 3 shows a sectional structure of a semiconductor laser according to a second embodiment of the present invention. As shown in the figure, a multiple quantum well (MQW) active layer 2 and an optical waveguide 5 are formed on an n-InP substrate 1 by a selective MOVPE method using a SiO ₂ mask, and the emission wavelength peaks are 1.55 μm and 1.2 μm, respectively. It is manufactured so that Next, 150 μm is applied to the active layer 2 of 450 μm.
A diffraction grating having a period of 0.2400 μm is provided in a region of 30 μm at intervals of μm, and a 125 μm
A sample diffraction grating 7 composed of a diffraction grating having a period of 0.2410 μm is formed in an area of 25 μm at intervals. Further, a p-InP cladding layer 3 is formed on the active region and the optical waveguide.
And an InP cladding layer 8 are formed by two growths. Thereafter, an optical waveguide having a stripe structure having a current blocking layer is formed according to a normal laser manufacturing process, and antireflection coatings 12 and 13 are formed on the n-side electrode 9, the p-side electrode 10, and the front and rear surfaces of the element, thereby manufacturing a laser. I do.

The oscillation and reflection spectra of the laser thus manufactured in the active region and the distributed reflector region constitute a vernier structure as in the first embodiment, and the temperature dependence is 0.10 nm / ° C. and 0.12 nm / °, respectively. ° C. When the element temperature is changed from 0 ° C. to 100 ° C., the oscillation wavelength of this laser periodically changes according to the same principle as in the first embodiment, and the distribution of the change is stabilized to 0.5 nm or less. .

Embodiment 3 FIG. 4 shows a sectional structure of a semiconductor laser according to a third embodiment of the present invention. The only difference from the first embodiment is that an integrated heater 14 made of titanium is integrated. FIG.
Shows the temperature dependence of the oscillation wavelength when the integrated heater of the manufactured laser is not operated. As the temperature rises, the wavelength continuously changes to the long wavelength side at about 0.1 nm / ° C. Every time the temperature changes about 8 ° C, the oscillation mode changes to the next one, and the oscillation wavelength moves discontinuously to the short wavelength side. I do. In the temperature range from 0 ° C. to 100 ° C., the distribution of the oscillation wavelength is stabilized to about 1 nm or less. FIG. 6 shows a case where the integrated heater is operated by injecting a current, and the oscillation wavelength is stabilized by finely adjusting the element temperature. Since the amount of temperature change due to heater heating may be 8 ° C or less, a maximum of about 5
Oscillation wavelength indicated by ● in the figure is obtained by current injection of 0 mA, and the oscillation wavelength distribution is stabilized to 0.1 nm or less.

[0019]

According to the laser structure having the distributed reflector according to the present invention, a laser element whose oscillation wavelength is stabilized against a change in element temperature is provided. As a result, a Peltier element or the like of a laser module becomes unnecessary, and Downsizing and cost reduction are realized.

[Brief description of the drawings]

FIG. 1 is a sectional view of a semiconductor laser according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of the present invention.

FIG. 3 is a cross-sectional view of a semiconductor laser according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view of a semiconductor laser according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of a relationship between a laser element temperature and a laser oscillation wavelength according to the present invention.

FIG. 6 is a diagram showing another example of the relationship between the laser element temperature and the laser oscillation wavelength of the present invention.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 n-InP substrate 2 active region 3 p-InP cladding layer 4, 5 optical waveguide 6, 7, 11 sample diffraction grating (sampled grating) 8 InP cladding layer 9 n-side electrode 10 p-side electrode 12 anti-reflection coating (front surface) 13) Anti-reflection coating (rear side) 14 Integrated heater 21 Oscillation wavelength 22 Reflection spectrum on front side of element 23 Reflection spectrum on rear side of element 24 Wavelength temperature gradient on front side of element 25 Wavelength temperature gradient on rear side of element

Claims (6)

[Claims] 1. A semiconductor laser having a Bragg reflector, comprising: at least two types of optical waveguides having different temperature dependences of equivalent refractive index; and a diffraction grating structure provided in the optical waveguide and reflecting a plurality of wavelengths. A semiconductor laser characterized by the above-mentioned. 2. The semiconductor laser according to claim 1, wherein the diffraction grating structure provided in the optical waveguide is a sample diffraction grating. 3. The optical waveguide forming the plurality of sample diffraction gratings, wherein a product of an equivalent refractive index of the optical waveguide and a diffraction grating period at least at an average temperature when an element temperature is changed is constant. 3. The semiconductor laser according to claim 1, wherein a reciprocal ratio between the temperature gradient of the refractive index change and the sample period of the sample diffraction grating is constant. 4. A distributed reflection type having at least an active region, a passive region having a temperature dependency of an equivalent refractive index different on both sides of the active region in a resonator direction, and a sample diffraction grating in the passive region. The semiconductor laser according to claim 3. 5. The semiconductor laser according to claim 3, comprising at least an active region having the sample diffraction grating and a passive region having the sample diffraction grating. 6. The semiconductor laser according to claim 4, further comprising a heating mechanism for controlling a temperature of said optical waveguide by current injection.