JPH02395A - Semiconductor laser device and wave-length changing type ultrashort light pulse generator - Google Patents

Abstract

PURPOSE:To obtain short wavelength laser beam pulses having a high output and in a very short time by conducting mode locking by TM mode oscillation and coupling beam pulses with an optical guide type wave-length changing element. CONSTITUTION:Currents are injected to the active layer 10 of a semiconductor laser 16 from a power supply 38. Laser beams 40 emitted from a first end face 14 on which an antireflection film 15 is shaped are passed through a lens 42, a polarizer 44, through which only a TM polarizing component is passed, and an etalon 46 as a wave-length controller, and reflected by an external reflector 22 and fed back to the semiconductor laser 16. When inrush currents are modulated by the round trip frequency f=2L/C (in formula, C represents light velocity and L a distance between the end face 12 of the laser 16 and the reflector 22) of an external resonator, beam pulses 48 for a time (1ps or less) shorter than conventional devices are acquired. It is because the residual reflectivity of the antireflection film 18 is reduced at a TM mode. The beam pulses can be coupled directly with a wave changing element, thus acquiring short wave-length pulses having a high output.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an ultrashort optical pulse generator using semiconductor laser mode-locking technology, which is applied to optical computing, optical information processing, optical measurement, optical communication, etc. It is related to.

BACKGROUND OF THE INVENTION The present invention is proposed for two main problems. One issue is that the T
Rather than mode locking in M-mode oscillation, the T
This is about the fact that mode locking in M-mode shooting is clearly more effective in generating ultrashort optical pulses. Another problem is to propose a wavelength conversion type ultrashort optical pulse generator that generates ultrashort optical pulses with short wavelengths using the first new technology described above.

First, the background of the first issue will be explained.

Generating optical pulses with a time width on the order of picoseconds using a semiconductor laser is an important technology for application to optical computers, optical information processing, optical measurement, and optical communications. It is a well-known fact among those in the industry that one of the methods of generating ultrashort optical pulses using a semiconductor laser is mode-locking technology that converts the semiconductor laser into an external co-container.This technology can be roughly divided into active mode-locking. (active mode-1ocking)
and passive mode lock
There are two types: king). These include, for example, J.P.

P. Vander Ziel, “Mode L
oking of semiconductor
La5ers Sem1 conductors
and Semimetals, vol, 22. P
art B, Chapter 1. Page 1
+ (1985). Active mode locking modulates the current injected into the semiconductor laser with the round trip frequency of the external cavity. Passive mode-locking is achieved by including a saturable absorber within the resonator. In either case, optical pulses are oscillated at the round trip frequency of the external resonator C/2L (here, C: speed of light, L: external resonator length), that is, at a time interval of 2L/C, and the time width of the optical pulse is 1.
~20 picoseconds. Estimating from the width of the oscillation spectrum, an optical pulse width that is almost Fourier transform limited is obtained.

By the way, as described above, in order to perform mode locking using a semiconductor laser, the semiconductor laser has an external resonator configuration.

Further, an antireflection film is applied to the external cavity side of the semiconductor laser end face.

The basic configuration of a conventional example is shown in FIG. An anti-reflection film 18 is applied on the first end face 14 of the semiconductor laser 16, which consists of an active layer 10 that performs stimulated emission by current injection, and first and second end faces 14 and 12 using crystal cleavage planes. ing. Laser light 20 emitted from this semiconductor laser 16 is reflected by an external reflector 22 disposed outside, and the reflected laser light 20 is returned to the semiconductor laser 16. Since the antireflection film 18 is applied to the first end face 14, the resonator of this laser is constituted by the second end face 12 on the side opposite to the external resonator and the external reflector 22. When the bias of the current injected into the semiconductor laser 16 is set below the threshold value and the current is modulated at the round trip frequency of the external resonator, the second end face 1 of the semiconductor laser 16
The laser beam 24 emitted from the laser beam 2 oscillates as a light pulse train 26 . This is active mode synchronization.

A typical single semiconductor laser oscillates in a TE mode in which the polarization characteristic of the output light is in a direction in which the electric field direction of the light is parallel to the active layer. Originally, a semiconductor laser can oscillate in a TM mode in which the electric field direction of its output light is perpendicular to the active layer, but it oscillates in a TM mode. This is true, for example, for T.

Ikegami, “Reflectivity of Mode at Facet and Oscillation Mode in Double”
Heterostraft Injection Lasers,
IEEE Journal of Quantum Electronics (T, Ikegami Ref 1ecti
vityst Facet and 0scil
lati.

n Mode in Double-Heter
.

5structure Injection La5e
rs IEEE Journal of Q
uantum Electronics, vol.
QE-8, rIh6P, 470 (1972). An excerpt from this document is shown in FIG. 10. Figure 10 is a diagram showing that two different orthogonal polarized lights, TE mode (solid line) and TM mode (dashed line), have different reflectances at the semiconductor laser end facet, and the horizontal axis is the This is the film thickness of the active layer, and the vertical axis is the intensity reflectance of the laser end face.The transverse mode is the 0th-order fundamental mode oscillation, and the refractive index of the active layer is 3.6.
represents the ratio of the refractive index difference between the cladding layer and the active layer. What can be seen from FIG. 10 is that the reflectance of the end face is considerably greater in the TE mode than in the TM mode, although it depends on the thickness of the layers constituting the laser, the difference in refractive index, etc. In other words, in a semiconductor laser in which both end faces are cleaved, the resonator reflectance is larger in the TE mode than in the TM mode, so the threshold gain required to cause stimulated emission is smaller. oscillate. The same applies not only to a single semiconductor laser but also to a mode-locked semiconductor laser. In the conventionally reported mode-locked semiconductor laser, as shown in FIG. The electric field oscillates in the TE mode 28 in the direction of vibration.

On the other hand, recently a polarizer was inserted into the cavity of a semiconductor laser,
Research is being conducted to control the polarization of light emitted from semiconductor lasers. For example, T. Fujita et al., “Volatization Switching in a Single Frequency External Cavity Semiconductor Laser, Electronics Letter”
l, “Po1arization switch
ing in a single frequency
cy ext6rnal cavity sem
iconductor 1aser elect
ronics Letters vol, 23P,
803 (1987)), T. Fujita et al., “Bolarization in External Cavity Semiconductor Lasers, Applied Physics Letters” (T., Fujita et al.
“Po1arization bistable
ity in external cavity
y semiconductor 1asers
, Applied physics Lett.
ers vol, 51. P, 392 (1987)
) is explained in detail. In these documents,
It has been shown that the semiconductor laser emitted light oscillates in the 1M mode. That is, as shown in FIG. 11, between the first end surface 14 and the external reflector 22 as the external resonator surface,
When a Glan-Thompson prism, for example, is inserted as the polarizer 30, the laser light 32 inside the resonator oscillates in the TM mode 34 in the electric field direction of the light. At this time, the polarizer 30 is placed in the direction through which the 1M mode passes. Therefore, the output laser beam 36 also oscillates in the 1M mode.

In order to generate ultrashort optical pulses by mode-locking,
It is necessary for the external resonant modes of a semiconductor laser to be lined up at precisely equal frequency intervals so that these many modes oscillate in the same phase. For this purpose, it is necessary to ideally reduce the reflectance of the output end face on the external resonator side, that is, the first end face, of the semiconductor laser having the external resonator configuration to zero. That is, as shown in the conventional example of FIG. 9, an antireflection film 18 is provided on the first end surface 14. However, in reality, it has been reported that when a semiconductor laser oscillates in the TE mode, the reflectance of this end face does not become completely zero, and that a certain amount of residual reflectance exists. When residual reflectance exists at this intermediate end facet, the mode spacing of each external resonant mode in the frequency spectrum of the oscillated laser beam is not equal.

The dependence of the external resonant mode spacing on the reflectance of the intermediate end face can be found, for example, in H. Sato et al.
etal, “Intensity fluct
uation insemiconductor
1asers c.

uploaded to external cavi
ty + IEEE Journal of
Quantum Electronics vo
1, QE-21, P46 (1985)'). This situation will be explained using FIG. 10. The frequency spectrum of light from a conventional mode-locked semiconductor laser oscillating in the TE mode as shown in FIG. 10 is as shown in FIG.
, +1≠ν −ν, and m+nmn−. Here, ν represents the oscillation frequency of each external resonator mode, and m and n are integers. In order to realize ultrashort optical pulse generation, as shown in FIG. 12(b),
A relational expression +l pan pan pan-1 is required, and although it is necessary for each spectrum mode to oscillate in synchronization with the same phase, it has not been realized yet. This is because, as described above, the effect of the residual reflectance of the intermediate end facet of the external cavity semiconductor laser is large.

Therefore, if each external resonance mode of the mode-locked semiconductor laser is made to oscillate at equal frequency intervals as shown in FIG. 12(b), it becomes possible to obtain even more ultrashort optical pulses.

Next, the background of the second problem will be explained.

Conventionally, in the short wavelength region of 0.6 μm or less,
As long as IV-V compound semiconductors are used, laser oscillation is impossible due to the band gap energy limit, and therefore there are no lasers that oscillate at short wavelengths, and large gas lasers are used. As a result, devices that use lasers have become extremely large, and their industrial applications are limited. If a compact short-wavelength light source, for example a light source in the blue wavelength range, were to be put into practical use, it would have an extremely large impact on information processing fields such as optical discs and laser printers, as well as all optical measurement fields. Therefore, devices that convert semiconductor laser light to half the wavelength using second harmonic generation are being researched. For example, Taniuchi and Dashi, "Miniatureized Light Source of Coherent Blue Radiation" Ta
n1uchi and K, Yamamoto;
“Min i turned light
5source of coherent b
lue radiation) Technical Digest
Digest of (LEOI87) wp6 (
(1987).

FIG. 13 shows the conventional LiNb0. 1 shows a configuration diagram of converting the wavelength of semiconductor laser light using an optical wavelength conversion element using an optical waveguide. The TE mode oscillation substrate wave 50 emitted from the semiconductor laser 16 is collimated by the lens 52 and
After being converted into the TM mode by the second plate 54, the light is condensed by the lens 56 and enters the leading waveguide 60 formed on the LtNbo 58. At this time, the phase velocity of the second harmonic 62 that is Cherenkov-radiated from the fundamental wave propagating in the optical waveguide 60 becomes equal, and the second harmonic is efficiently generated. Currently, when the optical output of a semiconductor laser with a wavelength of 0.84 μm is 120 mW, a second harmonic of about 1 mW with a wavelength of 0.42 μm is obtained.

In this conventional embodiment, a λ/2 plate 54 is used to convert the TE mode of the semiconductor laser emitted light into the TM mode. The reason for converting to TM mode is that LiNb0. This is because when the semiconductor laser is propagated through the optical waveguide 60 formed above, only the TM mode propagates effectively.

In this case, it would be possible to rotate the semiconductor laser by 90 degrees and make the TE mode oscillation look like TM mode oscillation, but there is no way to do that. This situation will be explained with reference to FIG. 14.

Figure 14 shows the arrangement of the semiconductor laser and the L i N b O3 optical waveguide, and the near-field pattern when the semiconductor laser is coupled to the end face of the optical waveguide.
and the direction of vibration of the electric field.

FIG. 14(a) simply shows the location where the semiconductor laser and the optical waveguide are arranged, and although the near-field images match, the semiconductor laser light does not propagate through the optical waveguide because the polarization directions are orthogonal.

FIG. 14(b) shows the conventional example shown in FIG.
By using the /2 plate, the semiconductor laser light propagates through the optical waveguide and SHG light is obtained.

FIG. 14(C) shows the case where the semiconductor laser is rotated by 90 degrees. At this time, the polarization directions match, but the near field pattern is distorted (does not match, and good results cannot be obtained).

Therefore, as shown in FIG. 14(d), if +α is technically added to the semiconductor laser and the near-field image and polarization direction are originally in the TM mode, the coupling efficiency increases and the device can be simplified.

On the other hand, the generation of ultrashort optical pulses is reported in "Picosecond generation by blue laser light source using float rSHG" by Taniuchi, Proceedings of the 49th Japan Society of Applied Physics Academic Conference 7a-ZD-8.

In the configuration shown in FIG. 13, a second harmonic with a half width of about 10 psec is obtained by a gain switch method in which the semiconductor laser is directly modulated by a rubber generator.

Problems to be Solved by the Invention As described in the above conventional technology, in mode locking in TE mode oscillation of a semiconductor laser, the residual reflectance of the intermediate end facet of the semiconductor laser is large, which hinders shortening of the pulse. There is.

Furthermore, in the wavelength conversion type short pulse generation, the second harmonic radiated by Cerenkov is collimated in the thickness direction of the waveguide, but is a diverging light with a spread angle of about 14° in the lateral direction. Therefore, if a pinhole or the like is used to focus the light on a point, the output will be too large, so increasing the output is a challenge for practical use. The challenge is to significantly improve wavelength conversion efficiency and further reduce the width of short pulse light.

The first object of the present invention is to provide a semiconductor laser with unprecedented TM technology.
This is to perform mode locking by mode firing and generate temporally short optical pulses.

A second purpose is to couple the mode-locked semiconductor laser emitting in the TM mode to a leading wavepath type wavelength conversion element to generate a light pulse with higher output and shorter time than the conventional one. Furthermore, the optical waveguide itself can be included as part of the mode-locked semiconductor laser.

Means to solve problems The configuration of the present invention is as described below.

a semiconductor laser having a first end face coated with an antireflection film; an optical system that converts laser light emitted from the first end face into parallel light; and returning the laser light to the first end face of the semiconductor laser. It has an external reflector as a resonator surface and a power source for injecting current into the semiconductor laser, and the laser resonator is constituted by the second end surface of the semiconductor laser and the external reflector, and the optical A polarizer for controlling polarization is arranged between the system and the external reflector so that only the TM mode of the laser light passes through, and the laser light emitted from the first and second end faces of the semiconductor laser is in the TM mode. This is an ultrashort optical pulse semiconductor laser device characterized by mode-locked oscillation in a mode.

Alternatively, a semiconductor laser whose first end face is coated with an antireflection film, an optical fiber that couples and guides laser light emitted from the first end face, and a semiconductor laser that couples and guides the laser light emitted from the first end face; The laser resonator includes an external reflector as a resonator surface that returns the current to the end surface of the semiconductor laser, and a power source for injecting current into the semiconductor laser. A polarizer configured to control polarization between the optical fiber and the external reflector is arranged so that only the TM mode of the laser light passes through, and is emitted from the first and second end faces of the semiconductor laser. The present invention is an ultra-short optical pulse semiconductor laser device characterized in that the laser light generated by the laser beam undergoes mode-locked oscillation in the TM mode. Further, an etalon as a wavelength controller may be disposed between the polarizer and the external reflector, and a diffraction grating having a wavelength control function may be used as the external reflector.

Alternatively, the above-described ultrashort optical pulse semiconductor laser device is provided with a structure in which the laser light emitted from the second end facet of the semiconductor laser is optically coupled to a wavelength conversion element having an optical waveguide structure, and the semiconductor laser The ultrashort optical pulse light that is mode-locked in TM mode and is emitted from the second end face of the TM mode is coupled to the wavelength conversion element as a fundamental wave, and the second harmonic of the fundamental wave is coupled as the output light from the wavelength conversion element. This is a wavelength conversion type ultrashort optical pulse generator that is characterized by generating waves.

Further, the semiconductor laser has a structure in which a semiconductor laser and a wavelength conversion element having an optical waveguide structure are optically coupled, and has a power source for injecting a current into the semiconductor laser, and the semiconductor laser has a structure in which a wavelength conversion element having an optical waveguide structure is optically coupled, and a power supply for injecting a current into the semiconductor laser. 1 has an anti-reflection film on one end face, and has a high reflection film on a second end face farther from the wavelength conversion element, and the wavelength conversion element has a fundamental wave on the first end face coupled with the semiconductor laser. and a high reflection film that functions as an external reflector to return the laser light coupled as a fundamental wave to the semiconductor laser on a second end face that is farther from the semiconductor laser, and the wavelength conversion This is a wavelength conversion type ultrashort optical pulse generator characterized in that the guided mode of the fundamental wave propagating through the device is the TM mode, and the semiconductor laser device performs mode-locked oscillation in the TM mode. At this time, the semiconductor laser element may be optically coupled to the wavelength conversion element by a becollimating lens and a focusing lens, or the semiconductor laser element and the wavelength conversion element may be coupled by a pad. Further, a distributed feedback semiconductor laser may be used as the semiconductor laser, and a saturable absorption region may be provided inside the semiconductor laser. Furthermore, even if the current injected from the power source into the semiconductor laser is modulated at the round trip frequency C/2L (C, speed of light) with respect to the optical distance 111L between the first end facet of the semiconductor laser and the external reflector. good.

Effects The present invention has the above-described configuration, and by controlling the polarization of the mode-locked semiconductor laser and causing it to oscillate in the TM mode where the residual reflectance of the intermediate end facet of the semiconductor laser is small, the intervals between the external resonator modes become equal. It becomes possible to generate ultrashort optical pulses that are shorter in time than conventional ones.

Further, when wavelength conversion is performed, the second harmonic is converted in proportion to the square of the output of the fundamental wave, so that it is possible to further shorten the pulse.

Example The present invention will be explained below using examples.

The figure shows an embodiment of the present invention. If the number used for each means is the same as the means used in the conventional example, the same number will be used in the description.

A current is injected from the power source 38 into the active layer 10 of the semiconductor laser 16 formed by the first and second cleavage end faces 14 and 12. First end surface 14 coated with anti-reflection film 18
The laser beam 40 emitted from the lens 42 is converted into parallel light by a polarizer 44, which serves as a polarization controller. The light passes through an etalon 46 as a wavelength controller, is reflected by an external reflector 22, and is optically returned to the semiconductor laser 16. At this time, if the polarizer 44 is arranged so that only the TM polarized component of the light emitted from the semiconductor laser 16 passes through, the laser light 40 will be in TM mode 3.
It oscillates at 4. When the injection current of this mode-locked semiconductor laser device is biased below the threshold current and the current is modulated at the round-trip frequency of the external resonator, the cleavage end face 1
The laser beam 48 emitted from 2 carries out the entire ultrashort optical pulse. In reality, the time width of the ultrashort optical pulse is determined by a streak camera or a second harmonic converter 2 such as Li1O.

It is possible to observe this using an autocorrelator using something like . If the external resonator length is 7.5 cm, the round drip frequency of the external resonator will be 2 GHz,
Active mode locking is achieved by modulating the injected current from the power source 38 at that frequency, and the pulse width of the emitted laser beam 48 becomes 1 picosecond or less. In other words, when mode-locked oscillation occurs, the external resonance modes line up at precisely equal intervals in the light spectrum as shown in Figure 12, and a relationship of 1ν −ν −R is obtained, ν9+n+i
Pan pan p n-l. That is, conventionally reported mode-locked semiconductor lasers do not have a polarizer 44 as a means for controlling polarization as shown in the present invention, so they always oscillate in the TM mode, and therefore the cleavage end facet 14 remains. The influence of the reflectance is large, and the spectrum of the oscillated light is such that the intervals between the external resonance modes are not equal (siffi +
n+I-man≠νn+. -ν and become n
-1, each external resonant mode is synchronized to the same phase, causing oscillation loss (and thus making it impossible to obtain ideal mode locking).

In contrast, the mode-locked semiconductor laser according to the present invention controls polarization and first oscillates in TM mode, making it possible to generate ultrashort optical pulses that are shorter in time than conventional lasers. .

FIG. 3 shows the pulse waveform measured by the autocorrelator when the semiconductor laser is mode-locked. Figure 3 (
a) and (b) are the case of mode locking in the conventional TE mode and the case of mode locking in the TM mode, respectively. As is clear from the figure, it is clear that mode locking in TM mode allows the width of the optical pulse to be made shorter.

FIG. 2 shows a second embodiment of a TM mode-locked semiconductor laser device using an optical fiber. This embodiment is conceptually the same as the first embodiment, and the lens optical system in the first embodiment is replaced with an optical fiber. That is, the laser light emitted from the first end surface 14 provided with anti-reflection III 18 is coupled to the optical fiber 64 and propagates through the optical fiber 64 . A polarizer 44 and an external reflector 22 are arranged on the end face of the optical fiber, and when the polarizer 44 is arranged so that only the TM acid component of the light emitted from the semiconductor laser 16 passes, the laser light 60 oscillates in the TM mode. do. When the current injected from the power supply 38 is biased below the threshold current and the current is modulated at the round trip frequency of the external resonator, the laser light 66 emitted from the second end face 12 becomes an ultrashort optical pulse oscillation. .

In the first embodiment of the present invention, an etalon is used as the wavelength controller, but any wavelength controller may be used. The diffraction grating combines the functions of a wavelength controller and an external reflector.

Semiconductor lasers include not only normal Fabry-Perot semiconductor lasers, but also lasers with distributed feedback (DFB) or distributed reflection (DBR) structures, and single-mode lasers with a complex resonator structure. For example, it may be an IPC laser (in this case, the wavelength controller 46 may not be included because it oscillates in a longitudinal single mode).Also, as a semiconductor laser material, an oscillation wavelength band of 0.7 to 0.8 μm may be used. A
Not only IGaAs-based and InP-based materials in the 1.2 to 1.6 μm band, but also any other materials may be used.

Further, in this embodiment, active mode locking is shown in which the injection current from the power supply 38 is modulated, but passive mode locking may be used in which the semiconductor laser has a saturable absorption region inside and is driven by a CW injection current.

Next, an embodiment of a wavelength conversion type ultrashort optical pulse generator will be described. In the first place, the wavelength conversion efficiency of a mode-locked semiconductor laser is (2N2
+1)/3N times.

Here, N is the number of synchronized longitudinal modes. This can be seen, for example, in F., Zarnick, J.

E, Mitsudwinter “Applied Nonlinear Obtics J (F, ZERNIKE andJ, E, M
IDWINTER″AppliedNonlinear
0ptics”) A WILEY-INTER8C
IENCE PUBLICATION p, 111
is described in detail.

In addition, the pulse width of the output optical pulse of a mode-locked semiconductor laser can theoretically be shortened to about 0.1 psec, which is suppressed to about 1/10 to 1/100 of that generated by the gain switch method. be done.

Therefore, with the configuration of the embodiment of the present invention described above, by wavelength converting the short pulse output of the mode-locked semiconductor laser, it is possible to increase the output of the second harmonic and shorten the pulse.

FIG. 4 shows a configuration diagram of a wavelength conversion ultrashort optical pulse generator according to a third embodiment of the present invention. 16 is wavelength 0.84μm
52, 56, 70 are lenses, 46 is an etalon plate, 22 is an external reflector, 44 is a polarizer, 58 is a wavelength conversion element, and 60 is an optical waveguide. An antireflection film 18 is applied to the first end face of the semiconductor laser 16 . The wavelength conversion element 58 is made of Z-Cut LiNbO.
An optical waveguide (width x thickness x length = 2μm x 0.4μ
m x 6 vm ), the fundamental wave is the lowest-order TM mode, and the harmonics are the TM radiation mode.

Semiconductor laser element 16. Lens 70 and external reflector 2
2 constitutes a mode-locked semiconductor laser, in which a laser beam 68 emitted from the first end face of the semiconductor laser element 16 is collimated by a lens 70 and passes through a polarizer 44 and an etalon plate 46 that pass only the TM mode. External reflector 2
After being reflected by the laser beam 2, it is returned to the semiconductor laser element 16 again.

The semiconductor laser element 16 is biased below the threshold current, and the modulation frequency f = C/2L (C is the speed of light).

By modulating the optical length between the end facet 18 and the external reflector 22 (L is the semiconductor laser), the mode spacing Δν=C/2L
Each mode oscillates in synchronization, and short pulse light 50 in the TM mode is generated.

The short pulse light 50 emitted from the second end face 12 of the semiconductor laser 16 is collimated by the lens 52 and enters the optical waveguide 60 by the lens 56 .

A part of the fundamental wave incident on the optical waveguide 60 has a wavelength of 0.42μ.
The wavelength is converted to the second harmonic of m and Cherenkov radiation 72
It is output to the board side as .

Since the intensity of the second harmonic is proportional to the square of the intensity of the fundamental wave, the pulse width of the pulsed light of the second harmonic is shorter than the pulse width of the fundamental wave. In this example, the pulse width is 10 pse.
A second harmonic of 7 psecC was obtained from the fundamental wave of c.

FIG. 8 shows a second harmonic optical pulse waveform measured by a streak camera. FIGS. 11(a) and 11(b) show the case where the semiconductor laser is gain switched and the case where the semiconductor laser is mode-locked in TM mode, respectively.

As is clear from the figure, it is possible to temporally shorten the pulse width by TM mode synchronization.

FIG. 5 shows a fourth embodiment of the present invention. In this embodiment, the TM mode-locked semiconductor laser using an optical fiber shown in the second embodiment is coupled to a wavelength conversion element.

Therefore, explanation of the mode-locked semiconductor laser portion will be omitted.

When the TM mode short pulse light 50 emitted from the second end surface 12 of the semiconductor laser 16 is coupled onto the optical waveguide 60 formed on the wavelength conversion element 58 via the lens 52.56, second harmonic light 72 is generated. is output to the substrate side as Cerenkov radiation.

What should be emphasized in the third and fourth embodiments of the present invention is that when compared with the conventional example shown in FIG. 5, they have the following advantages because they use mode locking in TM mode. λ/2
There is no need to use a plate, and high coupling between the semiconductor laser and the optical waveguide can be easily obtained. Even compared to the conventional gain switch method, the pulse width of the second harmonic light is shorter and higher output is possible.

FIG. 6 shows a configuration diagram of a wavelength conversion ultrashort optical pulse generator according to a fifth embodiment of the present invention.

The semiconductor laser 16 has an antireflection film 18 on its first end face, and a high reflection film 74 on its second end face. At the input end of the wavelength conversion element 58, there is an anti-reflection film 7 for the wavelength of the fundamental wave.
6, a high reflection film 78 for the fundamental wave is applied to the output end.

The emitted light 78 of the semiconductor laser 16 enters the wavelength conversion element 58 through the lens 52° 56, is guided through the optical waveguide 60, is reflected by the high reflection film 78, and returns to the semiconductor laser element 1.
He will be returned on 6th. In the optical waveguide 60, the TE mode becomes a radiation mode and is not guided, so only the TM mode is guided and fed back to the semiconductor laser element 16. Therefore, the semiconductor laser element 16 has a TM mode threshold gain smaller than that of the TE mode (
This results in 1M mode oscillation.

In this embodiment, a mode-locked semiconductor laser is constructed by using the end face of the wavelength conversion element 58 coated with the high reflection film 78 as an external reflector, and the semiconductor laser is biased below the threshold current to set the modulation frequency f, =C/2L (L is the end face 18.
By modulating the optical length (optical length between the end faces 78), short pulse light of the second harmonic can be obtained with a compact and stable configuration.

FIG. 7 shows a configuration diagram of a wavelength conversion ultrashort optical pulse generator according to a sixth embodiment of the present invention.

This embodiment is the same as the fifth embodiment except that the semiconductor laser element 16 and the wavelength conversion element 58 are coupled in a butt manner, but since no lens system is used, the apparatus is further miniaturized and optical axis alignment becomes easier.

As described above, in the third to sixth embodiments, LiNb0. Although the substrate was used, Li Tag,...
L t 10. , HIO3, KNbO3, or the like. Moreover, although phase matching was performed between the waveguide mode and the radiation mode, it may be performed between two waveguide modes. Of course, not only active mode locking but also passive mode locking may be used.

Effects of the Invention As described above, the ultrashort optical pulse semiconductor laser device of the present invention is a device using the unprecedented TM mode, and is capable of generating short optical pulses. Therefore, it has a high performance as a light pulse excavator, and is very effective as a reference clock light pulse for an optical computer, for example. Furthermore, since the time width of the optical pulse is small, a high bit rate is possible, and when combined with an external modulator, it is suitable as a light source for optical communication. Furthermore, compared to conventional devices, this ultrashort optical pulse semiconductor laser device can also be used as an optical frequency standard because the external resonance modes as a spectrum of light are arranged at precisely equal frequency intervals. be. In addition, by wavelength converting the short pulse light generated by mode-locking technology, it is possible to improve the conversion efficiency and make the pulse shorter than the wavelength conversion of the short pulse light generated by the conventional gain switch method. By configuring a reflector for mode locking on the output end face of the conversion element, it is also possible to miniaturize the device.

[Brief explanation of the drawing]

1 and 2 are schematic configuration diagrams of ultrashort optical pulse semiconductor laser devices in the first and second embodiments of the present invention, and FIG.
The figure is a characteristic diagram showing the short pulse measurement results in TE mode and the short pulse measurement result in TM mode measured with an autocorrelator, respectively.
Figure 8 shows a schematic configuration diagram of the wavelength conversion type ultrashort optical pulse generator in this embodiment, and shows the waveform of a short pulse obtained by the gain switch method measured with a streak camera and the short pulse obtained by the mode locking method in TM mode. Characteristic diagrams showing the pulse waveforms, Figure 9 is a schematic configuration diagram of a conventional TE mode mode-locked semiconductor laser, Figure 1O is a characteristic diagram showing the polarization dependence of the semiconductor laser end face reflectance, and Figure 11 is a diagram of the semiconductor laser. A structural diagram for activating a laser in TM mode. Figure 12 is a diagram showing the difference between TE mode and TM mode regarding the frequency distribution of the external resonant mode interval. Figure 13 is a conventional wavelength conversion ultrashort optical pulse generator. FIG. 14 is a diagram showing the arrangement of a semiconductor laser and a wavelength conversion element, the near-field pattern and polarization direction of the semiconductor laser, and the near-field pattern and polarization direction of light that can be guided in the wavelength conversion element. 10... Active layer, 12.14... Cleavage plane, 16... Semiconductor laser, 18.76...
...Anti-reflection film, 22...Reflector, 34.
...TM mode, 38...Power supply, 40.
48...Laser light, 42,52.56.70.
...Lens, 44...Polarizer, 46...
... Etalon, 64 ... Optical fiber 58
... Wavelength conversion element, 60 ... Optical waveguide, 74.78 ... High reflection film. Name of agent Patent attorney Shigetaka Awano and 1 other personFig. Figure 10 Decoration thickness (μm)

Claims (1)

Scope of Claims: (1) a semiconductor laser whose first end face is coated with an antireflection film; an optical system that converts laser light emitted from the first end face into parallel light; First semiconductor laser
an external reflector as a resonator surface that returns to the end face of the
having a power source for injecting current into the semiconductor laser;
The laser resonator is constituted by the second end facet of the semiconductor laser and the external reflector, and a polarizer for controlling polarization is provided between the optical system and the external reflector to control only the TM mode of the laser beam. An ultrashort optical pulse semiconductor laser device, characterized in that the laser light emitted from the first and second end faces of the semiconductor laser is disposed so as to pass through the semiconductor laser and undergoes mode-locked oscillation in a TM mode. (2) combining a semiconductor laser whose first end face is coated with an antireflection film and a laser beam emitted from the first end face;
It has a wave-guiding optical fiber, an external reflector serving as a resonator surface for returning the laser light to the first end facet of the semiconductor laser, and a power source for injecting current into the semiconductor laser. As a device, the second semiconductor laser
and a polarizer for controlling polarization between the optical fiber and the external reflector,
An ultrashort optical pulse semiconductor laser, which is arranged so that only the TM mode of the laser beam passes through, and wherein the laser beam emitted from the first and second end faces of the semiconductor laser performs mode-locked oscillation in the TM mode. Device. (3) The ultrashort optical pulse semiconductor laser device according to claim 1 or 2, wherein an etalon serving as a wavelength controller is disposed between the polarizer and the external reflector. (4) The ultrashort optical pulse semiconductor laser device according to claim 1 or 2, characterized in that a diffraction grating having a wavelength control function is used as the external reflector. (5) The ultrashort optical pulse semiconductor laser device according to claim 1 or 2 has a structure in which the laser light emitted from the second end face is optically coupled to a wavelength conversion element having an optical waveguide structure, and The ultrashort optical pulse light emitted from the second end surface of the laser device and subjected to mode-locked oscillation in the TM mode is coupled to the wavelength conversion element as a fundamental wave, and the wavelength conversion element is coupled to the wavelength conversion element as an output light from the wavelength conversion element. A wavelength conversion ultrashort optical pulse generator characterized by generating second harmonics. (6) The semiconductor laser device has a structure in which a semiconductor laser device and a wavelength conversion element having an optical waveguide structure are optically coupled, and has a power source for injecting current into the semiconductor laser device, and the semiconductor laser device is connected to the wavelength conversion device. an anti-reflection film on a first end surface coupled to the semiconductor laser device, and a high reflection film on a second end surface farther from the wavelength conversion element, and the wavelength conversion element has an antireflection film for the fundamental wave on the end face of the semiconductor laser device, and a high reflection film that functions as an external reflector for returning the laser light coupled as the fundamental wave to the semiconductor laser device on the second end face farther from the semiconductor laser device. A wavelength conversion type ultrashort optical pulse generator comprising: a waveguide mode of a fundamental wave propagating through the wavelength conversion element is a TM mode, and a semiconductor laser device performs mode-locked oscillation in the TM mode. (7) The wavelength conversion type ultrashort optical pulse generator according to claim 5 or 6, wherein the semiconductor laser device is optically coupled to the wavelength conversion element by a collimating lens and a focusing lens. (8) The wavelength conversion ultrashort optical pulse generator according to claim 5 or 6, wherein the semiconductor laser device is pad-coupled with a wavelength conversion element. (9) The ultrashort optical pulse semiconductor laser device according to claim 1 or 2, wherein a distributed feedback semiconductor laser is used as the semiconductor laser. (10) The ultrashort optical pulse semiconductor laser device according to claim 1 or 2, which has a saturable absorption region inside the semiconductor laser. (11) The current injected from the power supply into the semiconductor laser
, its round-trip frequency C/2L(C;
3. The ultrashort optical pulse semiconductor laser device according to claim 1, wherein the ultrashort optical pulse semiconductor laser device is modulated at a speed of light. (12) The wavelength conversion ultrashort optical pulse generator according to claim 5 or 6, wherein a distributed feedback semiconductor laser device is used as the semiconductor laser device. (13) The wavelength conversion type ultrashort optical pulse generator according to claim 5 or 6, which has a saturable absorption region inside the semiconductor laser device. (14) The round trip frequency C/2 of the current injected from the power source into the semiconductor laser device with respect to the optical distance L between the first end facet of the semiconductor laser device and the external reflector
7. The wavelength conversion type ultrashort optical pulse generator according to claim 5 or 6, wherein the wavelength conversion type ultrashort optical pulse generator is modulated at L (C; speed of light).