JPH0964463A - Optical-pulse generation element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical-pulse generation element which can generate short optical pulses which are provided with a high-speed repetitive frequency and with a narrow time width by a method wherein a semiconductor laser is driven by a direct current, an optical phase modulator is operated at a large amplitude and the repetitive frequency of optical pulses is controlled. SOLUTION: In an MQW laser element 10, an optical waveguide 11 is formed on a substrate 20. The optical waveguide 11 is constituted of a semiconductor laser part 12 and of an optical phase modulator part 14 which is coupled to the semiconductor laser part. The semiconductor laser part 12 contains first semiconductor parts 24, 26, 28, 30, and the optical phase modulator part 14 contains second semiconductor parts 32, 34, 36. Then, the semiconductor laser part 12 is driven by a direct current, the optical phase modulator part 14 is operated at a large amplitude, an optical pulse train is generated, and the repetitive frequency of optical pulses is controlled by the optical phase modulator 14. Thereby, it is possible to generate short optical pulses which are provided with a high-speed repetitive frequency and with a narrow time width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generation element used as a light source in optical communication and optical measurement, and more particularly to an optical pulse generation capable of generating a short optical pulse having a high repetition rate and a narrow time width. Regarding the device.

[0002]

2. Description of the Related Art Semiconductor lasers have been used for the generation of long and short optical pulses because of their small size and direct modulation capability. The methods are roughly classified into three types: (1) Q switching method, (2) gain switching method, and (3) mode locking method. However, these methods have the following problems.

The Q-switching method has problems that the device is not easy to manufacture and the operation control is not easy.

The gain switching method requires a narrow and high repetition frequency current pulse source, although the method is simple and the repetition frequency is variable. In addition, the semiconductor laser itself must operate at high speed, and the optical pulse generated is a chirp characteristic peculiar to the semiconductor laser, so the product of the pulse width and its spectral width is several times larger than the value specified by the Fourier transform. There is a drawback that (the chirp amount is usually expressed by the line width expansion coefficient α,
This product is multiplied by (1 + α ² ) ^1/2 times using α). Therefore, it is necessary to drop most of the generated light spectrum with a filter or the like, which causes problems such as a decrease in light output and a complicated structure.

Although the mode-locking method has obtained a pulse width close to the theoretical limit, it requires a complicated external resonator structure, and the repetition frequency is defined by this external resonator structure, and the resonance frequency You can only get an integer multiple.

Recently, apart from the above three methods, a high-speed short optical pulse generation method using an electro-absorption type external modulator has been reported (reference: M. Suzuki et al.,
CLEO'92 Post Deadline Pap
er, CPD 26, pp. 56-57, 1992). FIG. 6 is a diagram for explaining the operation principle of the external modulator used for conventional pulse generation. That is, this figure shows the narrowing of the transmitted light intensity when the light intensity modulator is operated in a large amplitude with a sine wave voltage. In the electric field absorption type bulk modulator, the absorption coefficient changes as shown in the figure when an electric field is applied from the outside. As a result, it is possible to modulate the intensity of light transmitted through the modulator, the change of which is non-linear with respect to the applied voltage. Therefore, by irradiating continuous light from the outside (for example, moving a semiconductor laser with direct current) and mounting a sine wave signal on the modulator, a pulse of light narrower than a half wavelength of the signal can be generated. This method has an advantage that the repetition frequency can be freely set by using a sine wave voltage which is relatively easy to obtain, and an optical pulse having a narrow line width and a narrow spectrum width close to the Fourier transform limit can be obtained.

[0007]

However, the width of the generated optical pulse is limited by the voltage dependence of the light intensity transmitted through the external modulator and the band of the external modulator, and the band is 20 at most.
It is about GHz (8 to 10 GHz in the above document), the driving voltage is not low, and the intensity of light is weak due to the large coupling loss between the modulator and the optical fiber, which is a problem to be solved.

In order to overcome this drawback, there is an attempt to monolithically integrate the semiconductor laser and the light intensity modulator to reduce the coupling loss (for example, IEEE Journal).
alQuantum Electronics 29
Vol. Tanaka et al., 1993). However, there is a problem to be solved that satisfactory light intensity cannot be obtained.

In order to improve this, recently, the above-mentioned mode-locking method has been improved so that the semiconductor laser is operated by direct current, and a monolithically integrated optical intensity modulator is subjected to a large amplitude modulation at a resonance frequency to narrow it. There have been attempts to generate pulse trains of light (eg, Applied Physics Letter).
rs 1994). This method gives a large light output,
A narrow pulse width can also be obtained. However, since light absorption by voltage application is used, it is unavoidable to use a wavelength near the absorption edge in order to operate with a small voltage application. as a result,
There is some absorption even when no voltage is applied, which may act as supersaturated absorption. Further, since the absorber is present in the laser medium, the oscillation threshold is increased during laser operation, the quantum efficiency is reduced, non-uniform spectrum broadening is brought about, and the pulse width becomes wider than the Fourier transform limit. There are drawbacks. In addition, there is a problem to be solved that an optical pulse generated by this method is generally too narrow as a soliton light source (2 to 3 picoseconds at a repetition frequency of 10 GHz).

Therefore, the present invention solves the above-mentioned problems,
An object of the present invention is to provide an optical pulse generation element capable of generating a short optical pulse having a high repetition frequency and a narrow time width.

[0011]

In order to solve the above-mentioned problems, an optical pulse generating element according to the present invention is a semiconductor laser provided on a substrate and a semiconductor laser formed on the same substrate as the semiconductor laser. An optical phase modulator that phase-modulates the emitted light of the laser constitutes the same optical resonator, and
An optical pulse train is generated by driving the semiconductor laser with direct current and operating the optical phase modulator with a large amplitude,
The optical phase modulator controls the repetition frequency of the optical pulse.

Preferably, the semiconductor laser and the optical phase modulator are formed of the same optical waveguide, and the optical waveguide has a multiple quantum well structure having a quantum well layer and a barrier layer, and the quantum well is further provided. A tensile stress is introduced into the layer, while a compressive stress is introduced into the barrier layer, so that the polarization direction of the oscillation light of the semiconductor laser is TM.
Becomes

Further, preferably, the surface on the optical phase modulator side which constitutes the optical resonator is a highly reflective surface, so that light is extracted from the semiconductor laser side.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The optical pulse generating device of the present invention uses a semiconductor laser and an optical phase modulator using a multiple quantum well structure and having an ultrahigh speed (ultra wide band) and a low driving voltage. Also,
While maintaining characteristics such as small size and robustness, and without affecting the semiconductor laser itself, the transparency of the phase modulator itself is used, which makes it easy to control with a high repetition frequency, , Generate a high-power optical pulse train with a narrow width.

The optical pulse generating element of the present invention monolithically integrates an optical phase modulator and a semiconductor laser. This is because the semiconductor laser, which is the light source, and the optical phase modulator are composed of separate elements, and when they are combined, it is necessary to interpose an optical fiber between them. Therefore, a coupling loss occurs and the light intensity decreases. In addition, since a module is required for coupling with the optical fiber, not only the number of steps for manufacturing the element is increased, but also the reliability is deteriorated. Therefore, in the present invention, the end facet of the semiconductor laser and the end facet of the optical phase modulator constitute an optical resonator, a mode peculiar to this optical resonator is generated, and a large amplitude having a frequency corresponding to the mode interval is generated. A sine wave of is added to the optical phase modulator (called mode-locking). The light generated at this time is a narrow optical pulse with a width of a few picoseconds (psec) reflecting the nonlinearity and wide band property of the phase characteristic with respect to the voltage of the phase modulator. In principle, it is widened by √2, so it is practically useful as a soliton light source.
It will be 6 psec. A semiconductor optical phase modulator is provided on the same substrate, and the refractive index of the optical waveguide layer is changed through a voltage applied to the semiconductor optical phase modulator to change the equivalent optical path length of the optical resonator to change the resonance condition. The optical phase modulator employs a multiple quantum well structure (MQW), which is faster than the bulk type and is advantageous for low voltage driving, and the phase modulator utilizes a large change in the refractive index with respect to the applied voltage. At this time, in the optical phase modulator, as shown in FIG. 7, the absorption characteristic and the phase characteristic change depending on the applied voltage, so that the operating condition in which the refractive index change is large in a range where absorption does not occur is selected. Moreover, it has been found from our research that a large electric field effect can be obtained by applying a tensile stress to the well layer and a compressive stress to the barrier layer, and further, a larger refractive index change can be obtained by using TM polarized light. Obtained (Figure 7).

Even if the semiconductor laser itself is used to generate a narrow optical pulse by the Q switching or gain switching method, its line width has a large product with the spectral line width due to its chirp characteristic (usually, the chirp amount is It is represented by the line width expansion coefficient α, and α is 2 to 6 in the semiconductor laser,
This product is increased by (1 + α ² ) ^1/2 times using α), but since an external modulator with a small line width expansion coefficient α (0.2 to 1.0) is used, the line width is close to the theoretical limit. A light pulse having a narrow spectral width can be obtained. A modulator using a multiple quantum well structure, which has the highest performance of 40 GHz in 3 dB band, has been reported (Reference: Odaka et al., IEICE Transactions C-1, J74-C-1, Volume 1, No. 1).
1, pp. 414-420, November 1991), its broadband property has been proven. If the band is wide, the modulation frequency can be increased accordingly and the width of the optical pulse can be narrowed. At this time, the α phase parameter of the optical phase modulator is larger than that of the absorption type modulator, and the line width is about √2 times larger, but this width is appropriate as a soliton light source. Further, the optical intensity modulator needs to be driven with a large amplitude operation, but the optical phase modulator having the multiple quantum well structure requires only a small driving voltage, and the burden on the high-frequency signal source is reduced. Further, as shown in FIG. 8, in the multiple quantum well structure, the phase change changes non-linearly with respect to the modulation voltage, and the degree of the change is larger than that of the bulk modulator described above. A narrow light pulse can be generated even under irradiation with CW light, but the light intensity itself cannot be increased. If an optical phase modulator is inserted inside the laser resonator, the refractive index, that is, the phase of light, changes with the voltage from the outside, the resonance condition inside the resonator is changed, and the Only laser oscillation becomes possible, and a high-power sharp optical pulse can be generated according to an external voltage. At this time, if the TM is changed, a large change in the refractive index can be obtained with a small applied voltage under a small absorption loss (see FIG. 7).

FIG. 7 shows these relationships.
Compared to polarized light, TM polarized light is obtained under the condition of absorption loss with a small change in refractive index under the same applied voltage and detuning energy (energy difference between absorption edge and used light).

Further, the advantage of monolithic integration with a semiconductor laser is effective in improving the output of an optical pulse, and the problem with the method using the external modulator is that the light source and the modulator are connected via an optical fiber. The loss that is increased by combining the two can also be reduced. Further, the optical pulse train generated by the above method differs from an absorption type intensity modulator that utilizes a change in absorption coefficient by an applied voltage, has almost no loss, a laser oscillation spectrum is uniform, and there is no supersaturated absorption effect, and therefore jitter is also generated. There is little effect, there is no pattern effect, and the desired pulse waveform can be taken out to the outside. At this time, there is some chirping, but there is no problem because this can be compensated using the dispersion compensating fiber.

A configuration example of the optical pulse element structure of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a perspective view for explaining a schematic structure of an MQW laser device which is an example of an optical pulse device.
2 is a sectional view taken along the line II-II in FIG. In the figure, reference numeral 10 is an MQW laser element, 11 is an optical waveguide, 12 is a semiconductor laser section, 14 is an optical phase modulator section, 1
5 is a first semiconductor portion, 16 is a second semiconductor portion, 17
Is an optical coupling region, 18 is an interface, 20 is a substrate, 22 is an etching stop layer, 24 is a first lower cladding layer, 26 is an active layer (first multiple quantum well structure), 26A is a well layer,
26B is a barrier layer, 28 is a guide layer, 29 is a high reflection film,
30 is the first upper cladding layer, 31 is the protective layer, and 32 is the second
Lower cladding layer, 34 is a second multiple quantum well structure, 34
A is a well layer, 34B is a barrier layer, 36 is a second upper cladding layer, 38 is a third cladding layer, 38A is a thin portion, 3
8B and 38C are clad layer portions, 40 is a separation portion, and 42
Is a cap layer, 44, 46 and 47 are buried portions, 48 is an electrode of a laser portion, and 49 is an electrode of an optical phase modulator portion.

The MQW laser device 10 of the present invention has an optical waveguide 11 provided on a substrate 20. This optical waveguide 1
1 comprises a semiconductor laser section 12 and an optical phase modulator section 14 coupled to the semiconductor laser section 12. Semiconductor laser unit 12
Includes a first semiconductor portion 15 and includes an optical phase modulator portion 14
Includes a second semiconductor portion 16. The above-described first and second semiconductor portions 15 and 16 are provided on the substrate 20 via the etching stop layer 22. The first semiconductor portion 15 has a first lower cladding layer 24, an active layer 26 (first multiple quantum well structure), a guide layer 28, and a first upper cladding layer 30, which are stacked in this order. . The first upper and lower cladding layers 30 and 24 are doped so as to have different conductivity types, and sandwich the active layer 26 and the guide layer 28. Active layer 2
Reference numeral 6 constitutes a first multiple quantum well structure including a well layer 26A and a barrier layer 26B. Optical phase modulator unit 14
Is composed of the second semiconductor portion 16, the quantum well is slightly thin and the absorption edge wavelength is short wavelength, and the applied voltages can be applied independently of each other.

On the other hand, the second unit included in the optical phase modulator unit 14
Has a second lower cladding layer 32, a second multiple quantum well structure 34, and a second upper cladding layer 36,
They are laminated in this order. Second upper and lower cladding layers 36, 3
Similarly to the first upper and lower cladding layers, 2 is doped to have different conductivity types and sandwiches the second multiple quantum well structure. The second multiple quantum well structure 34 is similar to the first multiple quantum well structure in the well layer 34A and the barrier layer 3.
It consists of 4B. The semiconductor laser section 12 is optically coupled in the optical coupling region 17 facing each other through the lower cladding layer 32 of the optical phase modulator section 14.

A continuous clad layer 38 is provided on the first and second semiconductor portions 15 and 16, and the clad layer 38 forms an interface 1 between the first and second semiconductor portions 15 and 16.
8 has a separating portion 40 whose upper portion is cut off along a region including the opposing end portions 15A and 16A sandwiching 8 therebetween. That is, the interface 18
The laser portion 12 and the portion 38C of the clad 38 existing in the optical phase modulator portion 14 are integrally connected to each other by a thin portion 38A that straddles. The optical waveguide 11 has a ridge structure that penetrates the laser section 12 and the optical phase modulator section 14, and both sides of the laser section 12 and the optical phase modulator section 14 are respectively embedded sections 4.
4, 46 and 47 are provided at the same height as the cap layer 42. The cap layer 42 and the embedded portions 44, 46,
An electrode 48 of the laser section and an electrode 50 of the optical phase modulator section 14 are provided on the 47, respectively. The isolation between the laser section 12 and the optical phase modulator section 14 is improved by the separating section 40.

The MQW laser device described above can be manufactured as follows. That is, molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOVPE) is previously performed.
The laser section (or optical phase modulator section) fabricated on the substrate is selectively etched to the substrate by dry and wet etching methods, and then the optical phase modulator section (or laser section) is MBE method or MOVPE method. Grow using. At this time, the height of the laser emitting portion measured from the substrate surface should match the height of the optical waveguide portion of the optical phase modulator portion measured from the substrate surface. The laser section and the optical phase modulation section are capable of independently applying a voltage, and the absorption edge wavelength of the quantum well is set to a shorter wavelength than that of the laser by using a selective growth method, a mixed crystal technique or the like.

The manufacturing method of one embodiment of the optical pulse generating device of the present invention will be described below more specifically with reference to the drawings. FIG. 3 is a cross-sectional view for explaining the method of manufacturing the MQW laser device shown in FIGS. 1 and 2, and corresponds to the coupling portion between the semiconductor laser section 12 and the optical phase modulator section 14 in FIG. Further, in the figure, (A) to (F) represent each step.

The device structure shown in FIGS. 1 and 2 was manufactured as follows.

That is, M is formed on the surface of the n-InP substrate 20.
An n-InGaAsP layer is provided as an etching stop layer 22 by the OVPE method, an n-InP clad layer 24 is formed thereon to have a thickness of 0.1 μm, and 10 nm of InGaAs is used as a well layer (well layer) 26A, and an I corresponding to a wavelength of 1.3 μm.
Active layer 26 consisting of 6 layers of quantum well structure using nGaAsP 10 nm as barrier layer (barrier layer) 26B, wavelength 1.3 μm
An InGaAsP guide layer corresponding to m was grown to a thickness of 0.1 μm. A p-InP clad layer 30 was grown on it (FIG. 3 (A)).

After that, a SiO ₂ film 52 is formed by a sputtering apparatus, and a hole is formed in a desired portion by a photolithography technique. That is, lithography is performed using a two-layer mask with the patterned photoresist 54 (FIG. 3B). In the figure, reference numeral 54 is a patterned resist, and 56 is a SiO ₂ film eaves.

Next, using this as a mask, the optical phase modulator 14
Are grown by the MBE method or the MOVPE method (see FIG. 3).
(C)). At this time, selective growth is performed using a selective mask, and the optical phase modulator portion 14 is grown to be an InGaAs well layer having a thickness of 6.5 nm to form the optical waveguide 34, and the p-InAlAs cladding layer 36 is formed thereon. To form. A p-InGaAs protective layer 31 is formed on the clad layer 36.

Subsequently, the p-InP clad layer 38, p
-The InGaAs cap layer 42 is formed by the MOVPE method (FIGS. 3D to 3E).

Next, the active layer 26 and the optical waveguide layer 34 described above are etched using a stripe having a width of 1.5 to 3.0 μm to form a ridge penetrating the laser portion and the optical phase modulator portion. Thereafter, the laser portion is embedded with an InP layer (combined of p-InP and n-InP or semi-insulating InP) 44, and the optical phase modulator portion is embedded with a polyimide 47. Finally, an electrode 48, Attach 49 (FIG. 3 (F)).

Etching is performed using these electrodes 48, 49 and 50 as a mask to form a separating portion 40 between the electrodes to strengthen the insulation between the laser portion and the optical phase modulator portion.

The lengths of the laser portion, the optical phase modulator portion and the insulating portion between them were set to 2600 μm, 300 μm and 50 μm. A highly reflective coating (29) was applied to the end face of the optical phase modulator portion opposite to the laser side, and the emitting end face was set to the semiconductor laser side. Also, an n-type electrode 62 (FIG. 2) is provided on the lower surface of the substrate.
Turn on.

FIG. 4 is a cross-sectional view for explaining another embodiment of the method of manufacturing the MQW laser device shown in FIGS. 1 and 2. In this figure, the modulator portion is formed first. 4, the same reference numerals as those used in FIGS. 1 to 3 indicate the same members or portions. According to this figure, the light emitted from the semiconductor laser section 12 is guided to the optical phase modulator section 14.

FIG. 5 shows an MQW laser and an M to which the present invention is applied.
It shows the characteristics of the optical pulse generating element by the integrated light source of the QW optical phase modulator. A pulse width with a full width at half maximum of about 5 picoseconds was obtained at a repetition frequency of 20 GHz. Actually, according to the measurement of the autocorrelation by the second high frequency, the full width at half maximum is 1 picosecond assuming that the optical pulse waveform is Lorentz type. Met. The optical output is also several tens of mW, which is a sufficient value for this type of optical pulse. When a DC voltage was applied to the MQW optical phase modulator, the repetition frequency changed, and the intended purpose was achieved. At this time, the frequency of the RF signal applied to the intensity modulator must be changed according to the change of the repetition frequency.
Since the light returns through the modulator, the sample length is half that of a normal traveling waveform, and the same characteristics are obtained, so that a high extinction ratio and a high bandwidth are achieved.

As described above, according to the present invention, MQ
A light source in which the W laser and the MQW optical phase modulator are integrated efficiently can be easily manufactured, and the performance of each of the MQW laser and the MQW phase modulator can be optimized. A light pulse with a narrow line width and high repetition frequency and high output can be easily generated. In addition, the voltage applied to the MQW optical phase modulator is controlled so that the repetition frequency is several GH.
It can vary over z.

In the above, the semiconductor laser constituting the optical pulse generating section of the optical pulse generating element and the coupling form of the laser section and the modulator section have been described by using the structure called the semiconductor laser and the butt joint. Similar characteristics can be obtained even when the semiconductor laser included therein has a structure called LOG (see FIG. 4).

[0038]

As described above, the optical pulse generating element based on the present invention is a semiconductor laser provided on a substrate and a semiconductor laser formed on the same substrate as the semiconductor laser and emitting light from the semiconductor laser. An optical phase modulator that performs phase modulation constitutes the same optical resonator, further, the semiconductor laser is driven by direct current, and an optical pulse train is generated by operating the optical phase modulator with a large amplitude, and the optical pulse train is generated. Since the optical phase modulator controls the pulse repetition frequency, it is possible to generate a short optical pulse having a high repetition frequency and a narrow time width.

[Brief description of drawings]

FIG. 1 is an example of an optical pulse element according to the present invention M
It is a perspective view for explaining a schematic structure of a QW laser device.

FIG. 2 is a sectional view taken along line II-II in FIG.

FIG. 3 is a cross-sectional view for explaining the method of manufacturing the MQW laser device shown in FIGS. 1 and 2, which corresponds to the coupling portion between the semiconductor laser portion and the optical phase modulator portion in FIG. (A) to (F) of each represent each step.

FIG. 4 is a cross-sectional view for explaining another embodiment of the method of manufacturing the MQW laser device shown in FIGS. 1 and 2.

FIG. 5 is a diagram showing characteristics when the optical phase modulator of the optical pulse generating element according to the present invention is operated.

FIG. 6 is a diagram showing the operation principle of the optical phase modulator of the optical pulse generation element according to the present invention, and a diagram showing the effect of using TM polarized light.

FIG. 7 is a diagram showing the operation principle of the optical phase modulator of the optical pulse generating element according to the present invention, and is a diagram showing the refractive index and the amplitude change with voltage application (in the case of TE polarization).

FIG. 8 is a diagram for explaining the operation principle of a conventional intensity modulator used for pulse generation, showing a narrowing of transmitted light intensity when the light intensity modulator is operated at a large amplitude with a sine wave voltage.

[Explanation of symbols]

10 MQW Laser Element 11 Optical Waveguide 12 Semiconductor Laser Part 14 Optical Phase Modulator Part 15 First Semiconductor Part 15A Opposing End 16 Second Semiconductor Part 16A Opposing End 17 Optical Coupling Region 18 Interface (Oblique Etched Surface) 20 Substrate 22 Etching Stop Layer 24 First Lower Cladding Layer 26 Active Layer (First Multiple Quantum Well Structure) 26A Well Layer 26B Barrier Layer 28 Guide Layer 29 High Reflection Film 30 First Upper Cladding Layer 31 Protective Layer 32 Second Lower clad layer 34 Second multiple quantum well structure 34A Well layer 34B Barrier layer 36 Second upper clad layer 38 Third clad layer 38A Thin part 38B, 38C Clad layer part 40 Separation part 42 Cap layer 44, 46, 47 Embedded Section 48 Electrode for Laser Section 49 Electrode for Optical Phase Modulator Section 52 SiO ₂ Film 54 Patterned resist 56 eaves 62 n-type electrode

Claims (3)

[Claims] 1. An optical pulse generating element, comprising: a semiconductor laser provided on a substrate; and an optical phase modulator formed on the same substrate as the semiconductor laser and phase-modulating the emitted light of the semiconductor laser. However, the same optical resonator is configured, further, the semiconductor laser is driven by direct current, and an optical pulse train is generated by operating the optical phase modulator with a large amplitude,
An optical pulse generating element, wherein the optical phase modulator controls the repetition frequency of the optical pulse. 2. The optical pulse generating element according to claim 1, wherein the semiconductor laser and the optical phase modulator are formed of the same optical waveguide, and the optical waveguide includes a quantum well layer and a barrier layer. And a tensile stress is introduced into the quantum well layer, while a compressive stress is introduced into the barrier layer, whereby the polarization direction of the oscillation light of the semiconductor laser is TM. Optical pulse generator. 3. The optical pulse generating element according to claim 1, wherein the surface of the optical resonator on the optical phase modulator side is a highly reflective surface so that An optical pulse generating element characterized by extracting light.