JP3227701B2 - Mode-locked semiconductor laser - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a monolithically integrated mode-locked semiconductor laser used for optical communication, optical measurement, and optical information processing.

(Prior Art) Mode-locked semiconductor lasers are important in future ultrahigh-speed optical communication and optical measurement fields as small light sources capable of relatively easily generating ultrashort light pulses of several tens of ps or less. In a mode-locked semiconductor laser, a laser resonator is generally constituted by a laser chip serving as an active region, a reflecting mirror and a lens placed outside, and the like. Then, the drive current in the active region is modulated at a time period substantially equal to the round-trip time of the optical resonator, whereby forced mode locking is performed and an optical pulse is generated. However, such a mode-locked semiconductor laser having a hybrid configuration is not a practical light source in terms of mechanical stability.

Therefore, an attempt has been made to monolithically integrate an active region, a reflecting mirror, and an optical waveguide for optically coupling them into one semiconductor substrate. An example of such a monolithically integrated mode-locked semiconductor laser is the laser reported by RS Tucker et al. As shown in FIG. 7 (RSTucker et.al., Electronics Letters).
Electron. Lett. Vol. 25 pp. 621-623 (1989)). This laser has an active region 100 and an optical waveguide region 200 on an InP substrate 10.
Are integrated. The reflecting mirror 310 uses a cleavage plane of a crystal. In this laser, the active region 100 and the optical waveguide region 200 are a common light guide layer 20 transparent to laser light.
Optically coupled through The time width of this laser
An optical pulse of 4 ps and 40 GHz is obtained repeatedly. Another example of a monolithically integrated mode-locked semiconductor laser is the laser reported by PA Morton et al., In which a light pulse width is reduced by adding a saturable absorption region to a part of an optical waveguide (PAMorton et. al., Applied Physics Letters Appl. Phys. Lett. vol. 56 pp. 111-113 (19
90)).

(Problem to be Solved by the Invention) The above-described conventional example has the following problems. First, there is no function of controlling the spectrum width of the laser beam. Therefore, the spectrum width is unnecessarily widened at the time of modulation, and an optical pulse having a narrow spectrum width close to the transform limit cannot be obtained. As is well known, the spread of the spectral width is a major obstacle when transmitting an optical pulse through an optical fiber having dispersion. Therefore, it is important to control the spectrum width so that the spectrum width is the minimum with respect to the required time width of the light pulse. Conversely, controlling the time width of the light pulse is also an important function in application. This can be realized by controlling the spectrum width.

The second problem is that there is no function to control the resonator frequency. In general, in a forced mode-locked laser, in order to generate an ultrashort optical pulse, it is necessary to perform modulation at a frequency very close to a resonator frequency (difference of about 0.1% or less) which is a reciprocal of a light round-trip time. On the other hand, in optical communication, the clock frequency of the communication system is strictly predetermined. Therefore, when using a mode-locked laser for optical communication, it is necessary to strictly adjust the resonator frequency of the laser to the clock frequency. Even with monolithic integration, it is very difficult to control the actual resonator length with an accuracy of 0.1% or less during fabrication. Therefore, the function of controlling the resonator frequency, that is, the resonator length by some method becomes important.

The necessity of controlling the spectrum width of the mode-locked semiconductor laser and controlling the resonator frequency has been described above. When a mode-locked semiconductor laser is used for optical fiber communication, another effective function is control of wavelength chirp. The wavelength chirp of an optical pulse means that the central wavelength changes with time in the optical pulse. Generally, when an optical pulse is transmitted through an optical fiber, the time width of the optical pulse is widened due to the chromatic dispersion of the optical fiber, and the transmission characteristics deteriorate. This limits the distance over which transmission is possible. Therefore, if a wavelength chirp that cancels the dispersion is given to the optical pulse in advance, the influence of the dispersion is reduced and the transmission distance can be extended. The control of the wavelength chirp is a function of giving a wavelength chirp to the optical pulse of the mode-locked semiconductor laser so as to cancel the influence of dispersion. A mode-locked semiconductor laser having such a function has not been reported so far.

SUMMARY OF THE INVENTION An object of the present invention is to provide a monolithically integrated mode-locked semiconductor laser having a function of controlling a spectrum width, a resonator frequency or a wavelength chirp, which solves the above-mentioned problems in the conventional example.

(Means for Solving the Problems) A mode-locked semiconductor laser according to the present invention comprises: (1) an active region having a gain, an optical waveguide region transparent to oscillation light, and a diffraction grating on one semiconductor substrate. And the reflection region is optically coupled to the reflection region, and the reflection wavelength width of the reflection region is variable.

(2) An active region having a gain, an optical waveguide region transparent to oscillating light, and a reflection region having a diffraction grating are optically coupled and arranged on one semiconductor substrate. The optical length of the optical waveguide region is variable.

(3) An active region having a gain, an optical waveguide region transparent to oscillating light, a reflective region having a diffraction grating, and a non-linear region whose refractive index changes according to the intensity of oscillating light on one semiconductor substrate. Are optically coupled to each other.

(Example) Next, the present invention will be described with reference to the drawings.

FIG. 1 (a) is a perspective view showing a first embodiment of a mode-locked semiconductor laser according to the present invention, and FIG. 1 (b) is AA 'in FIG. 1 (a).
FIG. 3 is a structural sectional view taken along a line. The first embodiment is a mode-locked semiconductor laser capable of controlling the reflection wavelength width described in claim 1, and has a function of controlling the spectrum width of oscillation light. The laser has a gain active region 100, an optical waveguide region 200, and a reflective region having a diffraction grating 90.
It is made up of 300. The structure is based on a so-called distributed Bragg reflection laser. The laser cavity includes two reflectors, a distributed Bragg reflector in the reflective region 300 and a cleaved end face in the active region 100. The structural features are (1) that the electrode 80 is provided in the reflection region 300 and current can be injected into the light guide layer 20 in the reflection region 300; (2) the active region 100 and the optical waveguide region 200 That is, the resonator frequency corresponding to the combined resonator length matches the required period of the mode-locked optical pulse.
The diffraction grating 90 selectively reflects only light having a certain reflection wavelength width around the Bragg wavelength. For this reason, the spectrum width of the laser light is about this reflection wavelength width. The reflection wavelength width changes depending on parameters such as the length of the diffraction grating 90, coupling efficiency, and waveguide loss (or gain). Therefore, the spectrum width can be controlled by controlling any of these parameters. Also, the time width of the light pulse can be controlled by changing the spectrum width. In the first embodiment, the current injected into the light guide layer 20 in the reflection region 300 is controlled to change the waveguide loss and control the reflection wavelength width. This is schematically shown in FIG.

Hereinafter, the first embodiment will be described in detail while following the manufacturing procedure. First, a diffraction grating 90 having a period of 240 nm is formed on a part of the n-type InP substrate 10. Next, an n-type InGaAsP (λg = 1.3 μm) is formed on the substrate 10 by the first crystal growth.
Light guide layer 20, n-type InP stop layer 30, InGaAsP (λg =
1.55 μm) An active layer 40 and a p-type InP cladding layer 50 are sequentially formed. Next, the active layer 40 of the optical waveguide region 200 and the reflection region 300
Is selectively removed, and p-type is formed by the second crystal growth.
An InP cladding layer 51 and a p-type InGaAs cap layer 60 are sequentially formed. In order to control the transverse mode, an embedding structure is applied here. That is, after performing mesa-type etching along the optical axis of the laser, the InP high-resistance layer 70 is formed by the third crystal growth. All of the crystal growth uses the metal organic chemical vapor deposition method. Finally, the electrodes 80 are formed on the active region 100 and the reflection region 300. In order to electrically separate the active region 100 and the reflection region 300, the cap layer of the optical waveguide region 200 is removed. Active region 100, optical waveguide region 200, reflection region 30
The lengths of 0 are 250 μm, 1800 μm, and 500 μm, respectively. This mode-locked semiconductor laser generates a repetitive optical pulse of 20 GHz by modulating the drive current of the active region 100. By controlling the injection current in the opposite region 300, the spectrum width is controlled from 0.5 nm to 2 nm. Accordingly, the time width of the light pulse changes from 10 ps to 3 ps. The center wavelength of the oscillation light is 1.55 μm.

Next, a second embodiment will be described. As described above, the reflection wavelength axis can also be controlled by changing the length of the reflection area. The second embodiment is a mode-locked semiconductor laser that effectively controls the length of the reflection region, and has a function of controlling the spectrum width of the oscillating light, as in the first embodiment. FIG. 3 is a sectional view showing the second embodiment. The structure differs from the first embodiment in that the reflection region 300
In this case, the active layer 40 is provided similarly to the active region 100, and the electrode 80 of the reflection region 300 is divided into a plurality along the resonator axis direction. In the example of FIG. 3, the reflection area 300 is divided into three. Others are almost the same as the first embodiment. The method of effectively controlling the length of the reflection region 300 is as follows. Since there is an active layer 40 in the reflection region 300, if no current flows through the reflection region 300, the active region 1
Since the light emitted in 00 is completely absorbed in the reflection region 300, the effective length of the reflection region 300 can be regarded as zero. Therefore, in FIG. 3, when an electric current is applied only to the electrode 1 of the reflection region 300 so that the gain and the loss of this portion are equal to each other and it can be regarded as an almost transparent optical waveguide, the effective reflection region 300 The length is L1. Similarly, when a current flows through the electrode 1 and the electrode 2, the effective length of the reflection region 300 is L1 + L2. In this way,
By passing a current partially through the plurality of divided electrodes 80, the effective length of the reflection region 300 can be controlled. When the length of the reflection area 300 changes, the reflection wavelength width changes.
This is schematically shown in FIG. Since the manufacturing method is almost the same as that of the first embodiment, it will not be repeated here. The main difference is that the active layer 40 in the reflection area 300 is left. In the second embodiment, substantially the same characteristics as in the first embodiment can be obtained.

Next, a third embodiment will be described. This embodiment is similar to the optical waveguide region 20 described in claim 2.
A mode-locked semiconductor laser that controls the optical length of 0,
It has the function of controlling the laser cavity frequency. As described above, when a mode-locked semiconductor laser is used for optical communication, it is necessary to adjust the resonator frequency to the clock frequency extremely strictly (within about 0.1%).
FIG. 5 is a sectional view showing the third embodiment. The structure differs from that of the first embodiment in that the optical waveguide region 200 is provided with an electrode 80 separated from the electrode in the active region. The other points are almost the same as those of the first embodiment. By injecting a current into the light guide layer 20 of the optical waveguide region 200,
The optical length of the optical waveguide region 200 can be controlled by changing the refractive index of the light guide layer 20. Since the change in the refractive index due to current injection is about 1%, the effective resonator length is also set to 1%.
The degree can be changed. Thereby, the resonator frequency can be controlled. The manufacturing method is almost the same as in the first embodiment. The main difference is the formation of the electrode 80 in the optical waveguide region 300. When the length of each region is the same as that of the first embodiment, it is possible to generate a mode-locked optical pulse in a clock frequency range of 20 GHz ± 0.1 GHz. Note that the third embodiment can be combined with the first or second embodiment. The structure and manufacturing method in this case can be easily realized from the above description.

Next, a fourth embodiment will be described. This embodiment is a mode-locked semiconductor laser having a nonlinear region 200 in which the refractive index changes according to the intensity of the oscillating light, and has a function of controlling the wavelength chirp. FIG. 6 is a sectional view showing a fourth embodiment. The difference from the first embodiment in terms of structure is that the nonlinear region
This is the point where 400 and the second reflection area 310 are added. Here, a non-reflective coating film is formed on the end face of the nonlinear region 400. This nonlinear region 400 is the same as a so-called traveling wave type semiconductor optical amplifier. When a strong optical pulse is injected into a semiconductor optical amplifier, self-phase modulation occurs, and the wavelength of the optical pulse is temporally shifted from a long wavelength side to a short wavelength side. By controlling the size of this wavelength chirp according to the time width of the optical pulse, when transmitting the optical pulse through an optical fiber,
The effect of the spread of the time width of the light pulse due to dispersion can be reduced. The magnitude of the wavelength chirp is in the nonlinear region 40
It can be controlled by the current injected to zero. GP Agrowal et al. Have reported in detail about self-phase modulation in semiconductor optical amplifiers. (GPAgrawal et.al., IEE Journal of Quantum Electronics IEEE
J. Quantum Electron. Vol. 25, pp. 2297-2306 (198
9)). The second reflection area 310 has the same layer structure as the reflection area 300. However, this region is the same as the cleaved end face on the active region 100 side in the above-described embodiments, and only functions as one reflector of the laser resonator. Since the reflectance in the second reflection area 310 is set to be as low as about 10-20%, the reflection wavelength width of this area does not affect the spectrum width much. The manufacturing method of the fourth embodiment is the same as that of the first embodiment.
This is almost the same as the embodiment. Forming the non-linear region 400 and the second reflection region 310, the diffraction grating also in the second reflection region 310
The difference is that 90 is formed, and the anti-reflection coating film 95 is formed. The length of each region is 200 μm for the nonlinear region 400, 100 μm for the second reflection region 310, 250 μm for the active region 100, 3900 μm for the optical waveguide region 200 or 3900 μm, and 500 μm for the reflection region 300. This mode-locked semiconductor laser modulates the driving current of the active region 100 to repeatedly generate an optical pulse having a time width of 10 ps and a frequency of 10 GHz. By controlling the injection current in the nonlinear region 400, the wavelength chirp can be controlled. Non-linear region 400
The distance over which light pulses can be transmitted is 1.5
Can be expanded more than twice. Note that the fourth embodiment can be combined with the first, second, and third embodiments.

Hereinafter, some supplementary explanations of the above four embodiments will be given. In each of these examples, the oscillation wavelength is 1.55 μm.
In the case of the InGaAsP-based mode-locked semiconductor laser in the band, the same effect can be obtained with an AlGaAs-based laser in another wavelength band, for example, the 0.8 μm band. As the transverse mode control structure of the laser, another structure such as a ridge waveguide type can be applied. As for the manufacturing process, for example, there is a method in which the optical guide layer 20 in the optical waveguide region 200 is selectively crystal-grown. In this case, the light guide layer 20 of the active region 100 is not always necessary.

(Effect of the Invention) As described above, according to the present invention, a monolithic integrated mode-locked semiconductor laser capable of controlling the spectrum width, the resonator frequency, or the wavelength chirp can be realized.

[Brief description of the drawings]

FIG. 1A shows a first embodiment of a mode-locked semiconductor laser according to the present invention.
1 (b) is a cross-sectional view taken along the line AA 'of FIG. 1 (a), FIGS. 2 and 4 are diagrams for explaining the reflectance characteristics of the reflection region, and FIG. FIG. 5 is a sectional view of the element of the second embodiment, FIG. 5 is a sectional view of the element of the third embodiment, FIG. 6 is a sectional view of the element of the fourth embodiment, and FIG. It is sectional drawing of an element. In the figure, 100 is an active region, 200 is an optical waveguide region, 300 is a reflection region, 310 is a second reflection region, 400 is a non-linear region, 10 is a semiconductor substrate, 20 is a light guide layer, 30 is an etching stop layer, and 40 is Active layer, 50 and 51 are cladding layers,
Reference numeral 60 denotes a cap layer, 70 denotes a high-resistance layer, 80 denotes an electrode, 90 denotes a diffraction grating, 95 denotes a non-reflection coating film, and 1, 2, and 3 denote electrodes.

Claims (1)

(57) [Claims] An active region having a gain, an optical waveguide region transparent to oscillating light, and a reflection region having a diffraction grating are optically coupled and arranged on one semiconductor substrate. A mode-locked semiconductor laser, wherein a plurality of divided electrodes are provided in the reflection region.