JPH042190A - Mode-locked semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a monolithic integration type mode-locked semiconductor laser having a function of controlling a spectral width, a resonator frequency or a wavelength chirp by disposing an active region having a gain, an optical waveguide region transparent for an oscillation light, and a reflecting region having a diffraction grating by optically coupling on a semiconductor substrate. CONSTITUTION:A laser resonator has two reflectors, i.e., a distribution Bragg's reflector of a reflecting region 300 and a cleaved end of an active region 100. An electrode 80 is provided on the region 300, and a current can be injected to an optical guide layer 20 disposed on the region 300. A resonator frequency corresponding to a resonator length matched to the region 100 and an optical waveguide region 200 coincides with the period of a necessary mode synchronizing light pulse. A diffraction grating 90 selectively reflects only a light having a certain reflected wavelength width having a Bragg's wavelength as a center. Thus, the reflecting wavelength width is varied according to parameters such as the length of the grating 90, a coupling efficiency, a waveguide loss, etc. Accordingly, any of these parameters is controlled to control a spectral width.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a monolithically integrated mode-locked semiconductor laser used in optical communication, optical measurement, and optical information processing.

(Prior Art) Mode-locked semiconductor lasers are important in the future fields of ultrahigh-speed optical communication and optical measurement as compact light sources that can relatively easily generate ultrashort optical pulses of several Lops or less. In a mode-locked semiconductor laser, a laser resonator usually includes a laser chip as an active region, a reflecting mirror and a lens placed outside, and the like. By modulating the drive current of the active region with a time period approximately equal to the round trip time of light to the resonator, forced mode locking is performed and a light pulse is generated. However, such a hybrid configuration mode-locked semiconductor laser cannot be said to be a practical light source in terms of mechanical stability.

Therefore, attempts have been made to monolithically integrate an active region, a reflecting mirror, and an optical waveguide for optically coupling them onto a single semiconductor substrate.

An example of such a monolithically integrated mode-locked semiconductor laser is the laser reported by R,S, Tucker et al. as shown in FIG.

Tucker 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 integrated on an InP substrate 10. The reflecting mirror 310 utilizes the cleavage plane of the crystal. In this laser, the active region 100 and the optical waveguide region 200 are optically coupled through a common optical guide layer 20 that is transparent to the laser light. With this laser, optical pulses with a time width of 4 ps and a repetition rate of 40 GHz are obtained. Another example of a monolithically integrated mode-locked semiconductor laser is a laser in which a supersaturated absorption region is added to a part of the optical waveguide to narrow the optical pulse width, as reported by P. A. Morton et al. P., A., Marton et al.
, al, , Abride Physics Letters App1
．． Phys, Lett, vol, 56 pp,
111-11.3 (1990)).

(Problems to be Solved by the Invention) The conventional example described above has the following problems. First, there is no function to control the spectral width of laser light.

As a result, the spectral width becomes wider than necessary during modulation, making it impossible to obtain an optical pulse with a narrow spectral width close to the transform limit. As is well known, spectral width broadening is a major obstacle when transmitting optical pulses through dispersive optical fibers.

Therefore, it is important to control the spectral width so that it is the minimum spectral width for the required time width of the optical pulse. Conversely, controlling the time width of the optical pulse is also an important function for applications. This can be achieved by controlling the spectral width.

The second problem is that there is no function to control the resonator frequency. Generally, in forced mode-locked lasers, in order to generate ultrashort optical pulses, it is necessary to perform modulation at a frequency that is extremely close to the resonator frequency (with a difference of about 0.1% or less), which is the reciprocal of the round trip time of light to the resonator. be. On the other hand, in optical communications, the clock frequency of the communication system is strictly determined in advance.

Therefore, when a mode-locked laser is used for optical communication, it is necessary to precisely match the laser resonator frequency to the clock frequency. Even with monolithic integration, it is extremely difficult to control the actual resonator length with an accuracy of 0.1% or less during manufacturing. Therefore, the ability to control the resonator frequency, that is, the resonator length, in some way is important.

The necessity of controlling the spectral width and resonator frequency of a mode-locked semiconductor laser has been described above. When a mode-locked semiconductor laser is used for optical fiber communication, another effective function is wavelength chirp control. The wavelength chirp of a light pulse is a temporal change in the center wavelength of the light pulse. Generally, when optical pulses are transmitted through an optical fiber, the time width of the optical pulses is broadened due to wavelength dispersion of the optical fibers, resulting in deterioration of transmission characteristics. This limits the distance that can be transmitted. Therefore, if a wavelength chirp is applied to the optical pulse in advance to cancel out the dispersion, the influence of dispersion can be reduced and the transmission distance can be extended. Wavelength chirp control is a function that applies a wavelength chirp to the optical pulse of a mode-locked semiconductor laser that cancels out the effects of dispersion. A mode-locked semiconductor laser with 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 that solves the problems in the conventional example described above and has a function of controlling the spectral width, resonator frequency, or wavelength chirp.

(Means for Solving the Problems) The mode-locked semiconductor laser of the present invention has the following features: (1) An active region having a gain, an optical waveguide region transparent to oscillation light, and a diffraction grating are provided on one semiconductor substrate. and a reflective region having
It is characterized in that the reflection wavelength width of the reflection region can be varied.

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

(3) On one semiconductor substrate, an active region having a gain, an optical waveguide region transparent to the oscillated light, a reflective region having a diffraction grating, and a nonlinear region whose refractive index changes depending on the intensity of the oscillated light. are characterized in that they are arranged in an optically coupled manner.

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

FIG. 1(a) shows the first mode-locked semiconductor laser of the present invention.
FIG. 1(b) is a perspective view showing an embodiment of AA' in (a).
FIG. The first embodiment is a mode-locked semiconductor laser that can control the reflection wavelength width as set forth in claim 1, and has a function of controlling the spectral width of oscillated light. This laser is composed of an active region 100 having a gain, an optical waveguide region 200, and a reflection region 300 having a diffraction grating 90. 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. Structural features include (1) an electrode 80 is provided in the reflective area 300, and the light guide layer 20 in the reflective area 300 is provided with an electrode 80;
(2) a current can be injected into the active region 1;
The resonator frequency corresponding to the resonator length including the optical waveguide region 200 and the optical waveguide region 200 matches the period of the required mode-locking optical pulse. The diffraction grating 90 selectively reflects only light having a certain reflection wavelength width centered around the Bragg wavelength. Therefore, the spectral width of the laser beam is approximately equal to this reflection wavelength width. The reflected wavelength width changes depending on parameters such as the length of the diffraction grating 90, coupling efficiency, or waveguide loss (or gain). Therefore, the spectral width can be controlled by controlling any one of these parameters. Furthermore, the time width of the optical pulse can be controlled by changing the spectral width. In the first embodiment, the waveguide loss is changed by controlling the current injected into the light guide layer 20 of the reflection region 300, and the reflection wavelength width is controlled. This situation is schematically shown in FIG.

The first embodiment will be described in detail below while following the manufacturing procedure. First, a part of the n-type InP substrate 10 has a period of 24
A 0 nm diffraction grating 90 is formed. Next, by the first crystal growth, n-type InGaAsP is formed on this substrate 10.
(λg=1.3 pm) optical guide layer 20, n-type InP stop layer 30, InGaAsP (λg=1.55 pm) active layer 40, and p-type InP cladding layer 50 are formed in this order.

Next, the active layer 4 of the optical waveguide region 200 and the reflective region 300 is
After selectively removing 0, p
A type InP cladding layer 51 and a p-type InGaAs cap layer 60 are sequentially formed. In order to control the transverse mode, an embedded structure is applied here. That is, after mesa-shaped etching is performed along the optical axis of the laser, the InP high resistance layer 70 is formed by the third crystal growth. All crystal growth uses organometallic vapor phase epitaxy. Finally, active area 1
An electrode 80 is formed on the reflective region 300 and the reflective region 300. In order to electrically isolate the active region 100 and the reflective region 300, the cap layer of the optical waveguide region 200 is removed. The lengths of the active region 100, optical waveguide region 200, and reflection region 300 are 250 pm, 1800 pm, and 500 pm, respectively. This mode-locked semiconductor laser generates repetitive optical pulses of 20 GHz by modulating the drive current of the active region 100. By controlling the injection current in the opposite region 300, the spectral width is controlled from 0.5 nm to 2 nm.

Along with this, the time width of the optical pulse has changed from Lops to 3p.
It changes up to s. The center wavelength of the oscillated light is 1.55 pm.

Next, a second example will be described. As mentioned above,
The reflection wavelength axis can also be controlled by changing the length of the reflection region. The second embodiment is a mode-locked semiconductor laser that effectively controls the length of the reflection region, and similarly to the first embodiment, has the function of controlling the spectral width of the oscillated light. FIG. 3 is a sectional view showing the second embodiment.

Structurally, the difference from the first embodiment is that the reflective region 300 also has an active layer 40 like the active region 100, and that the electrode 80 of the reflective region 300 is divided into a plurality of parts along the resonator axis direction. It is that you are. In the example of FIG.
0 is divided into three parts. The rest is almost the same as the first embodiment. A method for effectively controlling the length of the reflective region 300 is as follows. Since the reflective region 300 includes the active layer 40, if no current is passed through the reflective region 300, all the light emitted from the active region 100 will be absorbed by the reflective region 300, so that the effective reflective region 30
A length of 0 can be considered zero. Therefore, in FIG. 3, a current is applied only to the electrode 1 in the reflective region 300,
When the gain and loss of this portion are made equal so that it can be regarded as a nearly transparent optical waveguide, the effective length of the reflection region 300 is Ll. Similarly, when a current is passed through electrode 1 and electrode 2, the effective length of reflection region 300 is Ll + L2. In this way, by partially passing current through the electrode 80 divided into a plurality of parts, the effective length of the reflective region 300 can be controlled. Reflection area 3
When the length of 00 changes, the reflected wavelength width changes. This situation is schematically shown in FIG. For the manufacturing method, see Part 1.
Since it is almost the same as the embodiment, it will not be repeated here. The main difference is that the active layer 40 in the reflective region 300 is left. In the second embodiment, almost the same characteristics as in the first embodiment can be obtained.

Next, a third example will be described. This example is
Optical waveguide region 200 described in claim 2
It is a mode-locked semiconductor laser that controls the optical length of the laser, and has the function of controlling the laser cavity frequency. As mentioned in the topic section, when a mode-locked semiconductor laser is used for optical communication, it is necessary to match the resonator frequency extremely closely (within about 0.1%) to the clock frequency.

FIG. 5 is a sectional view showing the third embodiment. The difference in structure from the first embodiment is that an electrode 80 is provided in the optical waveguide region 200, which is separate from the electrode in the active region. In other respects, this embodiment is almost the same as the first embodiment.

By injecting a current into the optical guide layer 20 of the optical waveguide region 200, the refractive index of the optical guide layer 20 can be changed and the optical length of the optical waveguide region 200 can be controlled. Since the refractive index change due to current injection is about 1%, the effective resonator length can also be changed by about 1%. This allows the resonator frequency to be controlled. Regarding the manufacturing method, the first
This is almost the same as the embodiment.

The main difference is the formation of the electrode 80 in the optical waveguide region 300. If the length of each region is the same as in the first embodiment, it is possible to generate mode-locked optical pulses within a clock frequency range of 20 GHz±0.1 GHz.

Note that the third embodiment can also be combined with the first or second embodiment. The structure and manufacturing method in this case are
This can be easily realized based on the explanation so far.

Next, a fourth example will be described. This example is
This is a mode-locked semiconductor laser that has a nonlinear region 200 whose refractive index changes depending on the intensity of oscillated light, as set forth in claim 3, and has a function of controlling wavelength chirp. FIG. 6 is a sectional view showing the fourth embodiment. Structurally, the difference from the first embodiment is that the nonlinear region 4
00 and the second reflective area 310 are added. Here, a non-reflection 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 shifts temporally from the long wavelength side to the short wavelength side. By controlling the magnitude of this wavelength chirp according to the time width of the optical pulse, it is possible to reduce the influence of the spread of the time width of the optical pulse due to dispersion when transmitting the optical pulse through an optical fiber. The magnitude of the wavelength chirp can be controlled by the current injected into the nonlinear region 400. Regarding self-phase modulation in semiconductor optical amplifiers, G, P,
Agrawal et al. have reported in detail. (G, P, A
Grawal et al., IEEE Journal of Quantum Electronics IEEE J.
Quantum Electron, vol, 25
．． pp. 2297-2306 (1989)). Second
The reflective region 310 has the same layer structure as the reflective region 300. However, this region is the same as the cleavage end face on the active region 100 side in the embodiments described above, and only functions as one reflector of the laser resonator. Since the reflectance in the second reflective region 310 is as low as about 10-20%, the reflected wavelength width of this region does not significantly affect the spectral width. The manufacturing method of the fourth embodiment includes the first
This is almost the same as the embodiment. The difference is that a nonlinear region 400 and a second reflective region 310 are formed, a diffraction grating 90 is also formed in the second reflective region 310, and a non-reflection coating film 95 is formed. The length of each region is the nonlinear region 40
0 is 200 pm, the second reflective area 310 is 10011 m,
Active region 100 is 250 pm, optical waveguide region 200 or 3
90011 m, and the reflective area 300 is 500 pm. This mode-locked semiconductor laser has a time width of Lops and a repetition rate of 10 GHz by modulating the drive current of the active region 100.
generates a light pulse of By controlling the injection current in the nonlinear region 400, the wavelength chirp can be controlled. Compared to the case without the nonlinear region 400, the distance over which optical pulses can be transmitted can be expanded by 1.5 times or more. Note that the fourth embodiment can also be combined with the first, second, and third embodiments.

Hereinafter, some supplementary explanations will be given regarding the above-mentioned four embodiments. In all of these examples, the oscillation wavelength is 1.55p.
Although this is an InGaAsP mode-locked semiconductor laser in the m-band, other wavelength bands, such as 0.8 μm band Al
A similar effect can be obtained with a GaAs-based laser. As for the transverse mode control structure of the laser, it is also possible to apply another structure such as a ridge waveguide type, for example. Regarding the manufacturing process, for example, the optical waveguide region 20
There is also a method of selectively growing crystals of the 0 photoactive layer 20. In this case, the light guide layer 20 in the active region 100 is not necessarily required.

(Effects of the Invention) As described above, according to the present invention, a monolithically integrated mode-locked semiconductor laser in which the spectral width, resonator frequency, or wavelength chirp can be controlled can be realized.

[Brief explanation of the drawing]

FIG. 1(a) shows the first mode-locked semiconductor laser of the present invention.
FIG. 1(b) is a perspective view showing an embodiment of AA' in (a).
2 and 4 are diagrams for explaining the reflectance characteristics of the reflective region, FIG. 3 is a sectional view of the element of the second embodiment, and FIG. 5 is a diagram of the third embodiment. FIG. 6 is a cross-sectional view of the element of the fourth embodiment, and FIG. 7 is a cross-sectional view of the conventional element. In the figure, 100 is an active region, 200
3 is an optical waveguide region, 300 is a reflective region, 310 is a second reflective region, 400 is a nonlinear region, 10 is a semiconductor substrate, 20 is an optical guide layer, 30 is an etching stop layer, 40 is an active layer, and 50.51 is a cladding layer. , 60 is the cap layer, 70
80 is an electrode, 90 is a diffraction grating, 95 is a non-reflective coating film, and 1.2.3 is an electrode.

Claims (3)

[Claims] (1) An active region having a gain, an optical waveguide region transparent to oscillated light, and a reflection region having a diffraction grating are optically coupled and arranged on one semiconductor substrate,
A mode-locked semiconductor laser characterized in that the reflection wavelength width of the reflection region is variable. (2) An active region having a gain, an optical waveguide region transparent to oscillated light, and a reflection region having a diffraction grating are optically coupled and arranged on one semiconductor substrate,
A mode-locked semiconductor laser characterized in that the optical length of the optical waveguide region is variable. (3) On one semiconductor substrate, an active region having a gain, an optical waveguide region transparent to the oscillated light, a reflective region having a diffraction grating, and a nonlinear region whose refractive index changes depending on the intensity of the oscillated light. A mode-locked semiconductor laser characterized in that these are arranged in an optically coupled manner.