JPH0793475B2 - Ultrashort optical pulse semiconductor laser device and wavelength conversion type ultrashort optical pulse generator - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultrashort optical pulse generator using a semiconductor laser mode-locking technology applied to optical computing, optical information processing, optical measurement, optical communication, and the like. It is a thing.

PRIOR ART The present invention is proposed for two main problems. One of the issues is the TE of the semiconductor laser that has been widely used from the past.
The proposed mode-locking in the TM-mode oscillation is clearly more effective in generating ultrashort optical pulses than the mode-locking in the mode-oscillation. Another object is to propose a wavelength conversion type ultrashort optical pulse generator that generates an ultrashort optical pulse of a short wavelength using the first new technique.

First, the background of the first problem will be described.

Generating an optical pulse with a time width of about picoseconds using a semiconductor laser is an important technique for applications to optical computers, optical information processing, optical measurement, and optical communication. As is well known to those skilled in the art, there is a mode-locking technique for converting one semiconductor laser into an external cavity by a method of generating an ultrashort optical pulse using a semiconductor laser. There are two types: mode-locking and passive mode-locking. These are, for example, JP Van der Ziel "Mode Locking of Semiconductor Lasers", Semiconductors and Semiconductors (JPVander Ziel, "Mo
de Loking of Somiconductor Lasers ", Semiconductors
and Semimetals, vol.22, Part B, Chapter 1, Page 1, (19
85). Active mode locking modulates the injection current to the semiconductor laser with the round-trip frequency of the external cavity. Passive mode-locking is performed by including a saturable absorber in the resonator. In either case, the optical resonator oscillates at a round trip frequency C / 2L (where C is the speed of light, L is the length of the external resonator) of the external resonator, that is, 2L / C, and the time width of the optical pulse is 1 It is about 20 picoseconds.
Estimating from the width of the oscillation spectrum, an almost Fourier transform limited optical pulse width 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.
An antireflection film is applied to the external cavity side of the end face of the semiconductor laser.

The basic structure of the conventional example is shown in FIG. An antireflection film 18 is formed on a first end face 14 of a semiconductor laser 16 composed of an active layer 10 for stimulated emission by current injection and a first and second end faces 14 and 12 using cleavage planes of crystals. ing.
The laser light 20 emitted from the semiconductor laser 16 is reflected by an external reflector 22 arranged outside, and the reflected laser light 20 is returned to the semiconductor laser 16. First end face 14
Since the antireflective film 18 is applied to the external resonator, the resonator of this laser has the second end face 12 on the side opposite to the external resonator and the external reflector.
It is composed of 22. If the bias of the injection current to 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 semiconductor laser
The laser light 24 emitted from the second end face 12 of 16 is an optical pulse train.
It oscillates as 26. This is a general active mode synchronization.

A normal single semiconductor laser oscillates in a TE mode in which the polarization characteristic of the output light is such that 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 TE mode. For example, T. Ikegami, “Reflectivity of Mode at Facet and Oscillation Mode in Double-Heterostructure Injection Lasers”, IEE Journal of Quantum Electronics (T.Ikegami “Ref
lectivityst Facet and Oscillation Mode in Double−
Heterostructure Injection Lasers ", IEEE Journal of
Quantum Electronics, vol-QE-8, No. 6P.470 (1972). The figures in this document are excerpted and
Shown in Figure 10. FIG. 10 is a diagram showing that two different orthogonal polarizations of the TE mode (solid line) and the TM mode (dashed line) have completely different reflectances at the end face of the semiconductor laser, and the horizontal axis shows the activity of the semiconductor laser. The layer thickness, and the vertical axis represents the intensity reflectance of the laser end face. The transverse mode is the 0th-order fundamental mode oscillation, the refractive index of the active layer is 3.6, and Δn 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 TE mode has a much higher reflectivity at the end face than the TM mode, although it depends on the layer thickness of the laser and the difference in refractive index. In other words, a semiconductor laser whose both end surfaces are cleaved has a higher resonator reflectivity in the TE mode than in the TM mode, so that the threshold gain required to cause stimulated emission can be small.
It oscillates in TE mode. The same applies not only to a single semiconductor laser but also to a mode-locked semiconductor laser,
As shown in FIG. 9, the conventionally reported mode-locked semiconductor laser has a laser beam 20 in which the oscillation direction of the electric field of the light is TE.
It oscillates in mode 28.

On the other hand, recently, a polarizer inside the cavity of a semiconductor laser was inserted,
Studies have been conducted to control the polarization of light emitted from a semiconductor laser. For example, T. Fujita et al., “Polarization switching in a single frequency-external cavity semiconductor laser”, Electronics Letter (T.Fujita et al., “Polarization
switching in a single frequency external cavity s
emiconductor laser ", Electronics Letters vol.23P.80
3 (1987)), T. Fujita et al. “Polaization bistability in external cavity semiconductor lasers”, Abride Physics Letters (T. Fujita et al., “Polarization bistability in
external cavity semicondactor lasers ", Applied phy
sics Letters vol.51, P.392 (1987)). In these documents, it is shown that the emitted light of the semiconductor laser oscillates in the TM mode. That is, as shown in FIG. 11, when a Glan-Thompson prism, for example, is inserted as the polarizer 30 between the first end face 14 and the external reflector 22 as the external cavity surface, the laser light 32 inside the cavity 32 is obtained. Oscillates in the TM mode 34 in the electric field direction of the light.
At this time, the polarizer 30 is arranged in the direction in which the TM mode passes. Therefore, the output laser light 36 also oscillates in the TM mode.

In order to generate an ultrashort light pulse by mode locking,
It is necessary that the external resonance modes of a semiconductor laser be aligned at exactly the same frequency intervals and that many of these modes oscillate in phase. For that purpose, it is necessary to ideally set the reflectance of the emission end face on the external cavity side of the semiconductor laser having the external cavity structure, that is, the first end face to zero. That is, as shown in the conventional example of FIG. 9, an antireflection film 18 is provided on the first end face 14. However, in reality, when the semiconductor laser oscillates in the TE mode, it is reported that the reflectance of this end face does not completely become zero and that there is some residual reflectance. When there is residual reflectance on this intermediate end face, the mode intervals of the external resonance modes of the frequency spectrum of the lasing laser light are not equal. For example, H. Sato et al. “Intensity fractionation
Semiconductor Lasers Coupled Toe External Gavity ", IEE Journal of Kandam Electronics (H.Sato eta
l, “Intensity fluctuation in semiconductor Iasers
coupled to external cavity "IEEE Journal of Quantum
Electronics vol.QE-21, P46 (1985)). This situation will be described with reference to FIG. 12th
The frequency spectrum of the light of a conventional mode-locked semiconductor laser that oscillates in the TE mode as shown in FIG.
And ν _{m + n + 1} −ν _{m + n} ≠ ν _{m + n} −ν _{m + n-1}
Become a relationship. Here, ν _m represents the oscillation frequency of each external resonator mode, and m and n are integers. In order to realize the generation of ultrashort optical pulses, the relational expression ν _{p + n + 1} −ν _{p + n} = ν _{P + n} −ν _{p + n-1} is necessary as shown in Fig. 12 (b). ,
At this time, each spectrum mode needs to be oscillated in synchronization with the same phase, but this has not been realized. This is because, as described above, the effect due to the fact that the reflectance of the intermediate end face of the external cavity semiconductor laser remains is large.

Therefore, if each external resonance mode of the mode-locked semiconductor laser oscillates at an equal frequency interval as shown in FIG. 12 (b), it becomes possible to obtain an ultrashort optical pulse.

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

Conventionally, in the short wavelength region of 0.6 μm or less, III
As long as a group-V compound semiconductor is used, laser oscillation is not possible due to the bandgap energy limit. Therefore, there is no laser that oscillates at a short wavelength, and a large gas laser is used. Therefore, the device using the laser becomes extremely large, and the field of industrial application is limited.
If a compact short-wavelength light source, for example, a light source in the blue wavelength range is put to practical use, it will have an extremely great impact on the information processing field such as optical disks and laser printers, and all optical measurement fields. Therefore, an element for converting the semiconductor laser light into a half wavelength by utilizing the second harmonic generation has been studied, and for example, Taniuchi, Yamamoto,
"Miniatureized Light Source of Coherent Blue Radiation" (T. Taniuchi and
K. Yamamoto; “Miniturized light source of ceherent
blue radiation) Technical Digest of (LEO´87)
Reported in wp6 (1987).

FIG. 13 shows a block diagram of wavelength conversion of a semiconductor laser light by an optical wavelength conversion element using a conventional LiNbO ₃ optical waveguide.
A substrate wave 50 of TE mode oscillation emitted from the semiconductor laser 16.
Is collimated by the lens 52, converted into the TM mode by the λ / 2 plate 54, condensed by the lens 56, and enters the optical waveguide 60 formed on the LiNbO ₃ 58. At this time, the fundamental waves propagating through the optical waveguide 60 have the same phase velocities of the second harmonics 62 radiated by Cherenkov, and the second harmonics are efficiently generated. Currently, the optical output of a semiconductor laser with a wavelength of 0.84 μm is 120 mW.
At, a second harmonic of about 1 mW having a wavelength of 0.42 μm was obtained.

In this conventional example, the λ / 2 plate 54 is used in order to convert the TE laser beam emitted from the semiconductor laser into the TM mode. The reason for converting to the TM mode is that when the semiconductor laser is propagated through the optical waveguide 60 formed on LiNbO ₃ , only the TM mode is effectively propagated.

Then, it is considered possible to rotate the semiconductor laser by 90 degrees and to see TE mode oscillation as if it were TM mode oscillation, but the method is not good. This situation will be described with reference to FIG.

FIG. 14 shows the arrangement of the semiconductor laser and the LiNbO ₃ optical waveguide, and the near-field pattern (near field pattern) and the vibration direction of the electric field when the semiconductor laser is coupled to the end face of the optical waveguide.

FIG. 14 (a) is a place where the semiconductor laser and the optical waveguide are simply arranged. Since the near-field images match but the polarization directions are orthogonal to each other, the semiconductor laser light does not propagate through the optical waveguide.

In FIG. 14 (b), as shown in the conventional example of FIG. 13, by using the λ / 2 plate, the semiconductor laser light propagates through the optical waveguide, and the SHG light can be obtained.

FIG. 14 (c) shows the case where the semiconductor laser is rotated by 90 °. At this time, the polarization directions match, but the near-field patterns do not match at all, and good results cannot be obtained.

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

On the other hand, the generation of ultrashort light pulses is reported in Taniuchi, Yamamoto, "Picosecond generation by blue laser light source using SHG", Proc. In the configuration shown in FIG. 13, the second harmonic having a half value width of about 10 psec is obtained by the gain switch method in which the semiconductor laser is directly modulated by the rubber generator.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention As described in the above-mentioned conventional technique, in mode-locking of the semiconductor laser in the TE mode oscillation, the residual reflectance of the intermediate end face of the semiconductor laser is large, which hinders shortening of the pulse. There is.

In the wavelength conversion type short pulse generation, the second harmonic emitted by Cherenkov is collimated in the thickness direction of the waveguide but divergent light with a divergence angle of about 14 ° in the lateral direction. Therefore, if a pinhole or the like is used to collect light at a point, much of the output is wasted, and higher output is an issue for practical use. Therefore, it is a subject to significantly improve the wavelength conversion efficiency of short pulse light and further reduce the width of short pulse light.

A first object of the present invention is to perform mode-locking of a semiconductor laser by TM mode oscillation which has not been used in the past, and to generate an optical pulse that is short in time.

A second object is to couple the mode-locked semiconductor laser of the TM mode oscillation to an optical waveguide type wavelength conversion element to generate a light pulse having a higher output and a shorter time than the conventional one. Further, the optical waveguide itself can be included in a part of the mode-locked semiconductor laser.

Means for Solving the Problems The constitution of the present invention is as follows.

A semiconductor laser having a first end face and a second end face, and an antireflection film formed on the first end face, and a laser which is on the first end face side and is emitted from the first end face. An optical system that makes light parallel light, and is on the opposite side of the first end surface with respect to the optical system,
An external reflector serving as a resonator surface for returning the laser light to the first end face of the semiconductor laser, and arranged between the optical system and the external reflector and passing only the TM mode of the laser light. A semiconductor laser, the optical system, and the polarizer.
A TM-mode laser light resonator is configured, and a resonator surface of the resonator is the second end surface of the semiconductor laser and the external reflector, and a current injected into the semiconductor laser is The optical distance L between the first end face of the semiconductor laser and the external reflector is modulated at the round trip frequency C / 2L (C; speed of light) and emitted from the second end face of the semiconductor laser. An ultrashort optical pulse semiconductor laser device in which laser light is mode-locked in TM mode.

Also, a semiconductor laser having a first end face and a second end face, and an antireflection film formed on the first end face; and a semiconductor laser on the first end face side, which is emitted from the first end face. An optical fiber that couples and guides a laser beam to the optical fiber, and an external resonator that is on the opposite side of the optical fiber from the first end face and that returns the laser light to the first end face of the semiconductor laser. A reflector, disposed between the optical fiber and the external reflector,
A polarizer that passes only the TM mode of the laser light is provided, and the semiconductor laser, the optical fiber and the polarizer constitute a resonator of the laser light of the TM mode, and the resonator surface of the resonator is Second of the semiconductor laser
Of the round-trip frequency C / 2L (for the optical distance L between the first end surface of the semiconductor laser and the external reflector). An ultrashort optical pulse semiconductor laser device in which the laser light emitted from the second end face of the semiconductor laser is modulated in C;

Further, the above-described ultrashort optical pulse semiconductor laser device and a wavelength conversion element arranged on the second end face side of the semiconductor laser device are provided, and the laser light emitted from the second end face of the semiconductor laser device is The ultrashort optical pulsed light that is optically coupled to an optical waveguide formed on the surface of the wavelength conversion element and is mode-locked in TM mode emitted from the second end face of the semiconductor laser device as a fundamental wave. A wavelength conversion type ultra-short optical pulse generator that is coupled to the optical waveguide of the wavelength conversion element and generates the second harmonic of the fundamental wave as the output light from the wavelength conversion element.

Further, the semiconductor laser includes a wavelength conversion element having an optical waveguide whose fundamental wave is TM mode is formed on the surface of the semiconductor laser, and the semiconductor laser reflects on a first end face coupled to the wavelength conversion element. The wavelength conversion element has an anti-reflection film and a highly reflective film on the second end face farther from the wavelength conversion element.
It functions as an antireflection film for the fundamental wave on the first end face that is coupled to the semiconductor laser, and as an external reflector that returns the laser light coupled as the fundamental wave on the second end face that is farther from the semiconductor laser to the semiconductor laser. A high reflection film, and the laser light emitted from the first end face of the semiconductor laser is coupled to the optical waveguide of the wavelength conversion element, and the guided mode of the fundamental wave propagating in the optical waveguide is TM mode. The semiconductor laser and the wavelength conversion element constitute a resonator for a laser beam of TM mode, and the resonator surface of the resonator is a highly reflective film formed on the second end surface of the semiconductor laser. A high reflection film formed on a second end face of the wavelength conversion element, wherein a current injected into the semiconductor laser causes an optical distance between the first end face of the semiconductor laser and the external reflector. With respect to the separation L, the semiconductor laser is modulated at its round trip frequency C / 2L (C; speed of light), and the semiconductor laser generates an ultrashort optical pulse that is mode-locked in TM mode. The ultrashort optical pulse is used as a fundamental wave. In the optical waveguide of the wavelength conversion element, there is provided a wavelength conversion type ultrashort optical pulse generator that generates the second harmonic of the fundamental wave.

Action The present invention has the above-described configuration, by controlling the polarization of the mode-locked semiconductor laser and oscillating in the TM mode with a small residual reflectance of the intermediate end face of the semiconductor laser, the external cavity mode intervals become equal, so It becomes possible to generate an ultrashort optical pulse that is shorter than the above.

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

Examples The present invention will be described below with reference to examples. An embodiment of the present invention is shown in the drawing. When the numbers used for the respective means are the same as those used in the conventional example, the same numbers will be used for description.

A current is injected from a power source 38 into the active layer 10 of the semiconductor laser 16 constituted by the first and second cleaved end faces 14 and 12. The laser light 40 emitted from the first end face 14 provided with the antireflection film 18 is collimated by a lens 42, passes through a polarizer 44 as a polarization controller and an etalon 46 as a wavelength controller, and It is reflected by the 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 oscillates in the TM mode 34. 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 laser light 48 emitted from the cleaved end face 12 oscillates an ultrashort optical pulse. . Actually, the time width of the ultrashort light pulse is
Streak camera or second harmonic converter, eg
It is possible to observe with an autocorrelator using something like LiIO ₃ . If the external resonator length is 7.5 cm,
When the round drip frequency of the external resonator is 2 GHz and the injection current from the power source 38 is modulated at that frequency, active mode locking is realized, and the pulse width of the emitted laser light 48 is 1 picosecond or less. That is, during mode-locked oscillation, as shown in FIG. 12b, the spectrum of light is such that the external resonance modes are precisely aligned and ν _{p + n + 1} −ν _{p + n} = ν _{p + n} −ν _{p + n-1}
It depends on the fact that That is, since the conventionally reported mode-locked semiconductor laser does not have the polarizer 44 as a means for controlling the polarization as shown in the present invention, it always oscillates in the TE mode, and therefore the cleaved end face 14 remains. The influence of the reflectance is large, and the spectrum of the oscillating light is such that the intervals between the external resonance modes are not equal as shown in FIG. 12 (a), and ν _{m + n + 1} −ν _{m + n} ≠ ν _{n + n} − Since it is ν _{m + n-1} , it is difficult to obtain ideal mode-locking because each external resonance mode is difficult to oscillate in synchronization with the same phase.
On the other hand, the mode-locked semiconductor laser according to the present invention controls polarization and oscillates in the TM mode for the first time,
It becomes possible to generate an ultrashort optical pulse that is shorter in time than before.

FIG. 3 shows a pulse waveform measured by the autocorrelator when the semiconductor laser is mode-locked. FIGS. 3 (a) and 3 (b) show the case of mode synchronization in the conventional TE mode and the case of mode synchronization in the TM mode, respectively.
As is clear from the figure, it is clear that mode-locking in the TM mode can shorten the width of the optical pulse.

FIG. 2 shows the case of a TM mode-locking semiconductor laser device using an optical fiber as the second embodiment. Conceptually the same as the first embodiment, the lens optical system in the first embodiment is replaced with an optical fiber. That is, the laser light emitted from the first end face 14 provided with the antireflection film 18 is coupled to the optical fiber 64 and propagates in the optical fiber 64. The polarizer 44 and the 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 component of the emitted light from the semiconductor laser 16 passes, the laser light 60 becomes TM.
It oscillates in the mode. 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. .

Although the etalon is used as the wavelength controller in the first embodiment in the present invention, any wavelength controller may be used. The diffraction grating also has a function as a wavelength controller and an external reflector. As the semiconductor laser, not only an ordinary Fabry-Perot type semiconductor laser but also a distributed feedback type (DFB) or distributed reflection type (DBR) structure laser, a single mode laser such as a compound resonator structure, for example, It may be an IPC laser, in which case the wavelength controller 46 may not be included because it oscillates in a longitudinal single mode. As a semiconductor laser material, the oscillation wavelength is 0.7 to
Not only AlGaAs-based material in the 0.8 μm band and InP-based material in the 1.2 to 1.6 μm band, but also any other material may be used.

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

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 (2N ² +
1) / 3N times increase. Here, N is the number of synchronized vertical modes. This means, for example, F. Zarnick, JE Midwinter “Applied Nonlinear
Optics "(F.ZERNIKE and JEMIDWINTER“ Appli
ed Nonlinear Optics ") A WILEY-INTERSCIENCE PUBLIC
ATION p.111 for details.

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

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

FIG. 4 shows a block diagram of a wavelength conversion ultrashort optical pulse generator according to a third embodiment of the present invention. 16 is AlGa with a wavelength of 0.84 μm
As semiconductor laser, 52, 56, 70 are lenses, 46 is etalon plate, 2
2 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 surface of the semiconductor laser 16. The wavelength conversion element 58 is a Z-Cut.
LiNbO ₃ substrate (size 2 × 2 × 6 mm) by optical waveguide by proton exchange (width × thickness × length = 2 μm × 0.4 μm × 6 m
m) is formed, the fundamental wave is the lowest TM mode, and the harmonic wave is the TM radiation mode.

The semiconductor laser device 16, the lens 70, and the external reflector 22 constitute a mode-locking semiconductor laser, and the laser light 68 emitted from the first end face of the semiconductor laser device 16 is collimated by the lens 70 and passes only the TM mode. After passing through the polarizer 44 and the etalon plate 46 and being reflected by the external reflector 22, it is returned to the semiconductor laser device 16 again. Semiconductor laser device
Biasing 16 below threshold current, modulation frequency f _m = C /
2L (C is the speed of light, L is a semiconductor laser) End face 18 and external reflector 22
By adjusting the optical length of the
Each mode of C / 2L oscillates in synchronism, and TM mode short pulse light 50 is generated.

Short pulse light emitted from the second end face 12 of the semiconductor laser 16
Lens 50 is collimated by lens 52, and optical waveguide 60 by lens 56.
Is incident on.

A part of the fundamental wave incident on the optical waveguide 60 is wavelength-converted into a second harmonic having a wavelength of 0.42 μm and output as Cherenkov radiation 72 to the substrate side.

Since the intensity of the second harmonic is proportional to the square of the intensity of the fundamental wave, the pulse width of the pulse light of the second harmonic becomes shorter than the pulse width of the fundamental wave. In the present embodiment, the second harmonic of 7 psec was obtained from the fundamental wave having the pulse width of 10 psec.

FIG. 8 shows the optical pulse waveform of the second harmonic measured by the streak camera. FIGS. 8A and 8B show a case where the semiconductor laser is gain-switched and a case where the semiconductor laser is mode-locked in the TM mode, respectively. As is clear from the figure, TM
It is possible to shorten the pulse width in time by mode-locking. FIG. 5 shows a fourth embodiment of the present invention.
In this embodiment, the TM mode-locking semiconductor laser using an optical fiber shown in the second embodiment is coupled to a wavelength conversion element. Therefore, the description of the mode-locked semiconductor laser portion is omitted. When the TM-mode short pulse light 50 emitted from the second end face 12 of the semiconductor laser 16 is coupled via the lenses 52 and 56 onto the optical waveguide 60 formed on the wavelength conversion element 58, the second harmonic light 72 is generated. Is output to the substrate side as Tierenkov radiation.

What should be emphasized in the third and fourth embodiments of the present invention is that the mode synchronization in the TM mode is used as compared with the conventional example shown in FIG. A high coupling between the semiconductor laser and the optical waveguide can be easily obtained without using a λ / 2 plate. Compared with the conventional gain switch method, the pulse width of the second harmonic light is short and high output is possible.

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

An antireflection film 18 is provided on the first end face of the semiconductor laser 16 and a high reflection film 74 is provided on the second end face thereof. An antireflection film 76 for the wavelength of the fundamental wave is provided on the entrance end of the wavelength conversion element 58, and a high reflection film 78 for the fundamental wave is provided on the exit end.

The emitted light 78 of the semiconductor laser 16 enters the wavelength conversion element 58 via the lenses 52 and 56, is guided through the optical waveguide 60, is reflected by the high reflection film 78, and is returned to the semiconductor laser element 16 again.
In the optical waveguide 60, the TE mode becomes the radiation mode, and since the TM mode is not guided, only the TM mode is guided and returned to the semiconductor laser device 16, so that the semiconductor laser device 16 has a TM mode threshold gain smaller than that of the TE mode. It becomes TM mode oscillation.

In this embodiment, the end face of the wavelength conversion element 58 provided with the high reflection film 78 is used as an external reflector to form a mode-locked semiconductor laser, and the semiconductor laser is biased to a threshold current or less and the modulation frequency f _m = C / 2L (L is the optical length between end face 74 and end face 78)
Harmonic short pulse light can be obtained.

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

The same as the fifth embodiment except that the semiconductor laser device 16 and the wavelength conversion device 58 are Babat coupled, but the apparatus is further downsized because the lens system is not used, and the optical axis alignment is facilitated.

Above, in the third to sixth embodiments, using a LiNbO ₃ substrate as the wavelength conversion _{element, LiTaO 3, LiIO 3, HIO} 3, KNbO 3
It may be a material such as. Further, the phase matching is performed between the guided mode and the radiation mode, but may be performed between the two guided modes. Of course, not only active mode synchronization but also passive mode synchronization 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 a TM mode which has not been available in the past, and can generate a short optical pulse. Therefore, it has been improved in performance as an optical pulse oscillator, and is very effective, for example, as a reference clock optical pulse for a computer. Further, since the time width of the optical pulse is small, a high pit rate can be achieved, and when combined with an external modulator, it is also suitable as a light source for optical communication. In addition, the present ultra-short optical pulse semiconductor laser device can be used as a frequency reference of light because the external resonance modes as an optical spectrum are arranged at precisely equal frequency intervals as compared with the conventional device. . In addition, by converting the wavelength of the short pulse light generated by the mode-locking technology, it is possible to improve the conversion efficiency and shorten the pulse compared to the wavelength conversion of the short pulse light generated by the conventional gain switch method. By configuring the mode-locking reflector at the emission end face of the conversion element, the device can be downsized.

[Brief description of drawings]

1 and 2 are schematic configuration diagrams of an ultrashort optical pulse semiconductor laser device according to the first and second embodiments of the present invention, respectively.
The figures are characteristic diagrams showing the short pulse measurement result in the TE mode and the short pulse measurement result in the TM mode, respectively, which are measured by the autocorrelator, and FIGS. 4 to 7 are the third to sixth embodiments of the present invention. Fig. 8 is a schematic diagram of the wavelength conversion type ultra-short optical pulse generator in, and Fig. 8 shows the waveform of the short pulse obtained by the gain switch method measured by the streak camera and the waveform of the short pulse obtained by the mode locking method of TM mode. FIG. 9 is a characteristic diagram showing the conventional TE mode mode-locked semiconductor laser, FIG. 10 is a characteristic diagram showing the deflection dependence of the end facet reflectivity of the semiconductor laser, and FIG. Configuration diagram for mode oscillation, FIG. 12 is a diagram showing the difference between TE mode and TM mode in the frequency distribution of the external resonance mode interval, FIG. 13 is a schematic diagram of a conventional wavelength conversion type ultrashort optical pulse generator, Figure 14 is semi-conductive Arrangement of the laser and the wavelength conversion element is a diagram showing a semiconductor laser of the near field pattern and polarization direction, the light can be guided light near-field pattern and the polarization direction of the wavelength conversion element. 10 ... Active layer, 12, 14 ... Cleaved surface, 16 ... Semiconductor laser, 18,76 ... Antireflection 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
...... Highly reflective film.

Claims (12)

[Claims] 1. A semiconductor laser having a first end face and a second end face, wherein an antireflection film is formed on the first end face, and the first end face on the first end face side. An optical system for collimating laser light emitted from the optical system, and the optical system is located on a side opposite to the first end face,
An external reflector serving as a resonator surface for returning the laser light to the first end face of the semiconductor laser, and arranged between the optical system and the external reflector and passing only the TM mode of the laser light. A semiconductor laser, the optical system, and the polarizer.
A TM-mode laser light resonator is configured, and a resonator surface of the resonator is the second end surface of the semiconductor laser and the external reflector, and a current injected into the semiconductor laser is The optical distance L between the first end face of the semiconductor laser and the external reflector is modulated at the round trip frequency C / 2L (C; speed of light) and emitted from the second end face of the semiconductor laser. Ultrashort optical pulse semiconductor laser device characterized in that laser light oscillates in TM mode. 2. A semiconductor laser having a first end face and a second end face, wherein an antireflection film is formed on the first end face, and the first end face on the first end face side. An optical fiber for coupling and guiding the laser light emitted from the optical fiber, and a resonator surface on the opposite side of the optical fiber from the first end surface for returning the laser light to the first end surface of the semiconductor laser. An external reflector as, and arranged between the optical fiber and the external reflector,
A polarizer that passes only the TM mode of the laser light is provided, and the semiconductor laser, the optical fiber and the polarizer constitute a resonator of the laser light of the TM mode, and the resonator surface of the resonator is Second of the semiconductor laser
Of the round-trip frequency C / 2L (for the optical distance L between the first end surface of the semiconductor laser and the external reflector). C: the speed of light), and the laser light emitted from the second end face of the semiconductor laser is mode-locked oscillating in the TM mode. 3. The ultrashort optical pulse semiconductor laser device according to claim 1, wherein the etalon as a wavelength controller is arranged between the polarizer and the external reflector. 4. The ultrashort optical pulse semiconductor laser device according to claim 1, wherein a diffraction grating having a wavelength control function is used as the external reflector. 5. A wavelength conversion type ultrashort optical pulse generator according to claim 1, wherein a distributed feedback type semiconductor laser is used as the semiconductor laser. 6. The wavelength conversion type ultrashort optical pulse generator according to claim 1, wherein a semiconductor laser having a supersaturated absorption region inside is used. 7. An ultrashort optical pulse semiconductor laser device according to claim 1 or 2, and a wavelength conversion element arranged on the second end face side of the semiconductor laser device, wherein the second semiconductor laser device comprises: A laser beam emitted from the end face is optically coupled to an optical waveguide formed on the surface of the wavelength conversion element, and is an ultra-short mode-mode oscillating TM mode emitted from the second end face of the semiconductor laser device. A wavelength conversion type ultrashort light, characterized in that optical pulsed light is coupled as a fundamental wave to an optical waveguide of the wavelength conversion element, and a second harmonic of the fundamental wave is generated as output light from the wavelength conversion element. Pulse generator. 8. A semiconductor laser, and a wavelength conversion element having an optical waveguide whose fundamental wave guided mode is a TM mode formed on a surface thereof, wherein the semiconductor laser is coupled to the wavelength conversion element. The wavelength conversion element has an antireflection film on an end surface and a high reflection film on a second end surface farther from the wavelength conversion element.
It functions as an antireflection film for the fundamental wave on the first end face that is coupled to the semiconductor laser, and as an external reflector that returns the laser light coupled as the fundamental wave on the second end face that is farther from the semiconductor laser to the semiconductor laser. A high reflection film, and the laser light emitted from the first end face of the semiconductor laser is coupled to the optical waveguide of the wavelength conversion element, and the guided mode of the fundamental wave propagating in the optical waveguide is TM mode. The semiconductor laser and the wavelength conversion element constitute a resonator for a laser beam of TM mode, and the resonator surface of the resonator is a highly reflective film formed on the second end surface of the semiconductor laser. A high reflection film formed on a second end face of the wavelength conversion element, wherein a current injected into the semiconductor laser causes an optical distance between the first end face of the semiconductor laser and the external reflector. With respect to the separation L, the semiconductor laser is modulated at its round trip frequency C / 2L (C; speed of light), and the semiconductor laser generates an ultrashort optical pulse that is mode-locked in TM mode. The ultrashort optical pulse is used as a fundamental wave. A wavelength conversion type ultrashort optical pulse generator, wherein the second harmonic of the fundamental wave is generated in the optical waveguide of the wavelength conversion element. 9. The wavelength conversion type ultrashort light according to claim 7, wherein the semiconductor laser device and the optical waveguide formed in the wavelength conversion element are optically coupled by a collimating lens and a focusing lens. Pulse generator. 10. The wavelength conversion type ultrashort optical pulse generator according to claim 7, wherein the semiconductor laser device and the wavelength conversion element are butt-coupled to each other. 11. The wavelength conversion type ultrashort optical pulse generator according to claim 7, wherein a distributed feedback type semiconductor laser is used as the semiconductor laser. 12. A wavelength conversion type ultrashort optical pulse generator according to claim 7, wherein a semiconductor laser having a supersaturated absorption region inside is used.