JPH0715083A - Collision pulse mode synchronous semiconductor laser - Google Patents

Abstract

PURPOSE:To generate a short optical pulse by making uniform the amplitude and waveform of electric field thereby realizing a stabilized CPM operation. CONSTITUTION:The collision pulse mode synchronous semiconductor laser comprises a first electrode 107 disposed in the center of the resonator of a multiple quantum well semiconductor laser 101 having Fabry-Perot structure, a plurality of second electrodes 102, 103, 104, 105 disposed symmetrically to the central part of the laser resonator while being separated, a third electrode 106 disposed at a position separated by a quarter of the length of laser resonator from the opposite edges of the laser, and means 110, 111, 112 for applying high frequency voltage to the third electrode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a short optical pulse generator used for high-speed optical communication and various optical measurements.

[0002]

2. Description of the Related Art FIG. 2 is a diagram showing the structure of a conventional collision pulse mode-locked semiconductor laser (CPM semiconductor laser), in which 201 is a reverse bias applying electrode, 202 is a forward bias applying electrode, 203 is a ground electrode, and 204 is a ground electrode. Is InGaAsP
/ InGaAs semiconductor laser active layer (optical waveguide layer), 2
Reference numeral 05 is an Fe-doped InP crystal growth layer, reference numeral 206 is N ⁺ In
P substrate, 207 is a high frequency applying electrode (M.C.W.
u, et al. , Appl. Phys. Lett. ，
vol. 8, pp. 759-761, 1990. ). FIG. 3 is a diagram showing the operation of this laser. Basically, the structure of this laser is such that the upper stripe electrode of an ordinary Fabry-Perot structure (FP) multiple quantum well semiconductor laser (MQW-LD) is divided and a part 201 thereof is applied with a reverse bias of a pn junction. The lower part of the optical waveguide layer is a saturable absorber, and high frequency is applied to some of the electrodes 207 to operate as a mode rocker. In the optical waveguide layer 204 of this laser, an optical pulse propagating in the right direction (FIG. 3A) and an optical pulse propagating in the left direction (FIG. 3).
(B)) exists.

When an electric signal is applied to the high frequency applying electrode 207, the loss (or gain) of the optical waveguide below the electrode 207 changes according to the amplitude of the applied electric signal. That is, by applying an electric pulse to the electrode 207, the lower part of the electrode can act as a shutter for a light pulse.

Now, when an electric signal is applied to the high frequency applying electrode 207 to open the shutter, an optical pulse propagates from the left and right ends of the laser toward the center. Since this laser has a bilaterally symmetrical structure, the waveforms, amplitudes, and velocities of both pulses are the same, and two pulses having the same waveform collide at the center of the laser. At this time, the frequency f of the high frequency signal applied to the electrode 207 is

[0005]

F = c / (nL) where c is the speed of light, n is the refractive index of the semiconductor, and L is the physical length of the laser, when an optical pulse that leaves one end face of the laser arrives at the other end face. Since the shutter on the other end face is opened, the condition for mode-locking operation is satisfied. Wu et al. Is 32.6G
Transform-limited short light pulses (1.4 ps) are obtained at a repetition rate of Hz.

The light saturable absorber below the electrode 201 is a medium in which the absorption loss of light decreases as the incident light intensity increases, and usually a dye or a semiconductor is used. Especially for light in the 1.55 μm wavelength band, multiple quantum well (MQ
The operation of the saturable absorber has been confirmed by using a semiconductor waveguide having a (W) structure with a reverse bias (specifically, YK Chen, MC Wu, T. Tanbu).
n-Ek, R.N. A. Logan and J. R. Si
mpson, “Monolithic collidi
ng-pulse mode-locked quan
tum well lasers, ”in Tec
h. Digest of Conference
Lasers and Electro-optics
(CLEO'91) No. CWK3, pp. 304-
307, 1991. See). Now, the standing wave of light is generated only at the moment when the pulses propagating in both directions collide in the saturable absorber, and the intensity of this standing wave of light becomes high, so the shutter action of the saturable absorber is sharp. Then, the optical pulse becomes steep. Regarding this principle, Yoichi Fujii
Nishizawa Koichi "Advanced Light Technology" pp. 110-129 (Agne Jofusha).

[0007]

The CPM semiconductor laser described above has the following problems.

In the electrode configuration of the conventional CPM semiconductor laser, the mode-locking high-frequency signal applying electrode is located at the outermost side of the laser resonator, that is, at both end surfaces. Therefore, the lower portion of the mode-locking high-frequency signal application electrode has air on one side and a semiconductor on the other side, and since the two have different dielectric constants, the electric field distribution due to the signal applied to the electrode is asymmetric. As the frequency increases, the electric field amplitude and waveform become spatially non-uniform due to the influence of the asymmetric electric field as compared with the case of a symmetrical electric field, which may lead to a decrease in mode-lock modulation efficiency.

The present invention has been made in view of such conventional problems, and the lower part of the mode-locking high-frequency signal applying electrode has a uniform structure to make the electric field amplitude and waveform uniform and stable. It is an object of the present invention to provide a means for realizing a CPM operation and generating a short optical pulse.

[0010]

In order to achieve the above object, the present invention provides a first structure provided on a central portion of a cavity of a multiple quantum well semiconductor laser having a Fabry-Perot structure.
An electrode, a plurality of second electrodes symmetrically divided with respect to the central portion of the laser resonator, and a third electrode provided at a position apart from both end faces of the laser by 1/4 of the laser resonator length. An electrode and a means for applying a high frequency to the third electrode are provided.

[0011]

According to the present invention, the mode-locking high-frequency signal applying electrode provided on the MQW semiconductor laser is arranged not at both end faces of the laser resonator but at a position apart from both end faces by 1/4 resonator length. The high-frequency signal for mode lock can be stably applied to the laser. That is, according to the present invention, the electrode of the MQW semiconductor laser is divided, and a part of the central portion of the resonator is reverse-biased so that the waveguide layer thereunder serves as an optical saturable absorber, and is located at a position of L / 4 from both ends of the resonator. By using the provided electrode as a modulation electrode and the other portion as an amplifying medium, a saturable absorber, a mode locker and an amplifying medium are monolithically integrated on the same semiconductor laser chip to generate a stable mode-locked optical pulse. Can be generated.

[0012]

EXAMPLES Examples of the present invention will be described below.

FIG. 1 shows a collision pulse mode-locked MQ according to the present invention.
1 is a diagram showing an embodiment of a W semiconductor laser device, and FIG.
(A) is a plan view seen from above, and (B) and (C) are diagrams for explaining the operation. In FIG. 1, 101 is MQ
W semiconductor laser, for example, n as in the conventional example of FIG.
⁺ InGaAsP / InGaA formed on InP substrate
sMQW active layer (optical waveguide layer), Fe-doped I that wraps it
Includes nP crystal growth layer. 102, 103, 104, 10
Reference numeral 5 is a DC electrode (stripe electrode), 106 is a high frequency signal applying electrode, 107 is a reverse bias applying electrode, and 108 and 10.
Reference numerals 9 and 110 denote a DC power source, 111 a bias T, 112 a high-frequency oscillator, and 113 and 114 mirrors that transmit a part of the light intensity. These two mirrors constitute a Fabry-Perot resonator and a semiconductor laser. The operation of is realized. As shown in the figure, the reverse bias applying electrode 107 is located in the center of the laser resonator, and the high frequency signal applying electrode 106
Is a position apart from both end faces by 1 / 4L when the resonator length is L, that is, the partial reflecting mirrors 113 and 114 and the electrode 107.
Has portions 106A and 106B located in the middle. The DC electrodes 102, 103, 104, 105 are not always necessary, and may be arranged symmetrically with respect to the center of the laser resonator. For example, only 103 and 104 or only 102 and 105 may be used. Below, Figure 1
The operation of the present laser will be described with reference to (B) and (C).

FIGS. 1B and 1C are views showing how an optical pulse propagates in a resonator in the present laser. Also in this laser, as in the prior art, an optical pulse propagating rightward in the laser resonator (FIG. 1B).
And a light pulse propagating to the left (FIG. 1C) exists. Now, generally, in a semiconductor laser, a pn junction is formed under the stripe electrode, and the bias electrode 102,
A forward bias current of about several tens of mA is supplied to the pn junction through 103, 104 and 105, and at the same time, a high frequency signal of about several tens mA is applied to the high frequency signal applying electrode 106 as a current amplitude. The frequency fm of this high frequency signal is

[0015]

Fm = 2c / (nL _LD ) L _LD is the total length of the laser of the present invention, where n is the refractive index of the semiconductor (≈3), this laser is also mode-locked by the same operating principle as the prior art. The motion will be satisfied. In general, the laser length of a semiconductor laser is about several 100 μm to several mm. The modulation frequency when the laser length is 10 mm is about 20 GHz.

Also in this laser, when a pn reverse bias of about several V is applied to the reverse bias applied voltage 107 located at the center of the laser, the waveguide layer portion 115 thereunder operates as a saturable absorber, and The standing wave of light is generated only at the moment when the pulses propagating in both directions collide in the saturable absorber 115, and the intensity of this standing wave of light becomes high. Therefore, the shutter action of the light saturable absorber 115 becomes sharp. Then the light pulse becomes steep. At this time, the partial reflecting mirrors 113, 11
If the transmittance of 4 is the same, the laser becomes completely symmetrical, and the optical pulse waveforms running left and right are the same, so that the collision pulse mode locking operation can be most efficiently realized. Equal or shorter light pulses can be generated. Further, in the laser of the present invention, the high-frequency signal applying electrode 106 is not at both end faces of the laser but at positions separated by L _LD / 4 from both end faces, and the lower part of the electrode 106 is a laser optical waveguide on both sides. Therefore, the shape of the electric field due to the high-frequency signal applied to 106 is bilaterally symmetric. Therefore, it is possible to realize stable mode-locking operation even in a higher frequency range without being affected by high frequency reflection at the laser end face and impedance mismatching as in the prior art.

[0017]

As described above, according to the present invention, the semiconductor laser 1 /
4 By arranging the high-frequency applying electrodes for mode locking at positions apart from each other by the laser length, the high-frequency signal is applied to the laser without being reflected by the end face, so that stable collision pulse mode-locking operation is realized even at high frequencies. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional technique.

FIG. 3 is a diagram illustrating an operation of a conventional technique.

[Explanation of symbols]

 101 MQW semiconductor laser 102, 103, 104, 105 DC electrode 106 High frequency applying electrode 107 Reverse bias applying electrode 108, 109, 110 DC power supply 111 Bias T 112 High frequency oscillator 113, 114 Partial reflector

Claims (1)

[Claims] 1. A first device provided on a central part of a cavity of a multiple quantum well semiconductor laser having a Fabry-Perot structure.
An electrode, a plurality of second electrodes symmetrically divided with respect to the central portion of the laser resonator, and a third electrode provided at a position apart from both end faces of the laser by 1/4 of the laser resonator length. A collision pulse mode-locked semiconductor laser device comprising an electrode and means for applying a high frequency to the third electrode.