JP2790536B2 - Ultrashort light pulse generator - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-short-wavelength semiconductor laser using a wavelength conversion technology applied to optical information processing, optical computing, optical applied measurement control field, and optical applied medical field. The present invention relates to an optical pulse generator.

2. Description of the Related Art As a method of generating an ultrashort pulse using a semiconductor laser, there are mainly an active mode locking method, a passive mode locking method, a gain switching method, and the like. Active and passive mode synchronization methods are described, for example, in Semiconductors and Semimetals, Volume 22, Part B, Chapter 1 (Semiconductors and sem
It is described in detail in imetals, Vol22, partB, Chapter1). The gain switching method is a method in which a semiconductor laser is excited by a short current pulse, a relaxation oscillation pulse generated as an excessive response of laser oscillation is extracted, and a short pulse light is generated. In this method, the repetition frequency can be set freely.

For the generation of a wavelength conversion type short optical pulse using a semiconductor laser and a waveguide type optical conversion element, see, for example, Taniuchi and Yamamoto, "Picosecond generation by blue laser light source using SHG"
It has been reported in the 9th JSAP Preprints 7a-zd-8. FIG. 5 shows a schematic configuration diagram of a conventional wavelength conversion type ultrashort optical pulse generator. 39 is a semiconductor laser, 32 is a wavelength conversion element, 33 is an optical waveguide, 34 and 35 are lenses, and 36 is a λ / 2 plate.

Fundamental wave of TE mode oscillation emitted from semiconductor laser 1
37 is collimated by a lens 34, the polarization direction is changed by 90 degrees by a λ / 2 plate 36, and then condensed by a lens 35 to be a wavelength conversion element.
The light enters the optical waveguide 33 formed in 32. At this time, the phase velocities of the fundamental wave 37 propagating through the optical waveguide 33 and the second harmonic 38 radiated by Cherenkov become equal, and the second harmonic is generated efficiently. A fundamental wave with a pulse width of about 20 ps is generated by performing gain switching of a semiconductor laser with a wavelength of 840 nm with a short current pulse having a pulse width of several hundred ps using a comb generator, and a second harmonic of a wavelength of about 420 ps with a pulse width of about 10 ps is generated. Has been obtained.

Problems to be Solved by the Invention As described in the above-mentioned conventional technique, in the active and passive mode-locking method of a semiconductor laser, the output light of the semiconductor laser is returned to the semiconductor laser again by an external resonator, so that the output is increased. There was a problem that can not be. In addition, the configuration of the apparatus is complicated and adjustment is difficult.

Although the semiconductor laser gain switching method can generate an ultrashort optical pulse with a simple configuration, if the peak of the current pulse is increased to increase the output, multiple pulses other than the first pulse of the relaxation oscillation oscillate. Is done. In the conventional technique, all of these plural pulses are made to enter the optical waveguide. Therefore, a plurality of pulses appear in the wavelength-converted second harmonic optical pulse waveform, and a single pulse which is desirable as an ultrashort optical pulse device cannot be obtained. Further, when a plurality of pulses are incident on the optical waveguide, the average value of the incident output is high, which causes optical damage in the waveguide, further deteriorating the characteristics.

The present invention has been made in view of the above problems, and even if the peak of a short current pulse for gain switching of a semiconductor laser is increased, the second harmonic light pulse obtained by wavelength conversion is a single pulse. It is another object of the present invention to provide an ultrashort optical pulse generator having a high output.

Means for Solving the Problems In order to solve the above-mentioned problems, an ultrashort optical pulse generator according to the present invention comprises: a semiconductor laser; a lens system for converting laser light emitted from the semiconductor laser into parallel light; A half-wave plate that rotates the polarization direction of light by 90 degrees, a condenser lens system that collects the parallel light, and a waveguide-type light conversion element that has a waveguide that converts the wavelength of the laser light, A semiconductor laser is driven by a pulse current, and a laser beam emitted from the semiconductor laser is a pulse light, and only a first pulse of relaxation oscillation by the semiconductor laser among the pulses is a waveguide-type optical converter having the waveguide. Since the laser light is incident on the element, the laser light can be wavelength-resolved in the focal length direction in the condenser lens system. Alternatively, a semiconductor laser and a lens system for converting the laser light emitted from the semiconductor laser into parallel light, a half-wave plate for rotating the polarization direction of the laser light by 90 degrees, a condensing lens system for condensing the parallel light, and the laser It has a waveguide type light conversion element having a waveguide that converts the wavelength of light, drives the semiconductor laser with a pulse current, and laser light emitted from the semiconductor laser is pulsed light, and the Since only the first pulse of the relaxation oscillation by the semiconductor laser is incident on the waveguide-type optical conversion element having the waveguide, the waveguide-type optical conversion element has a laser light incident surface on a first end face of the element. And a converging grating is provided between the incident surface and the portion that performs the wavelength conversion. A high-reflectance film is applied to both end faces of the optical waveguide formed in the wavelength conversion element, and the resonator type optical wavelength conversion element may be used, or the condensing grating may be formed on a half-wave plate. The semiconductor laser further includes a resonator including at least an active layer and a confinement layer, and a part or all of the resonator is a gain region having an electrode for injecting a current into the active layer. The other part may be a saturable absorption region.

Effect Due to the wavelength characteristics of the semiconductor laser light when the semiconductor laser is excited by a short current pulse, a difference in light wavelength occurs between the first pulse of relaxation oscillation and a plurality of subsequent pulses. FIG. 2 shows the relationship between the optical pulse waveform and its wavelength. In the present invention, the first pulse and subsequent pulses are spatially separated in the focal direction by a condenser lens system, and the optical waveguide formed on the wavelength conversion element is adjusted to the focal length position of the wavelength of the first pulse. By causing only the optical power of the first pulse to enter the optical waveguide, a single-pulse light with high output is output as the second harmonic, so that the output is output as a single pulse. For light measurement having time resolution such as fluorescence lifetime measurement, if it is not a single pulse light, the return pulse adversely affects the measurement as noise, so that the light source is suitable for such an application. Further, since the average output of the fundamental wave in the optical waveguide can be minimized, optical damage in the optical waveguide due to high average power is prevented, and the device operates stably.

Embodiment 1 FIG. 1 shows a schematic configuration of a wavelength conversion type ultrashort optical pulse generator according to a first embodiment of the present invention, 9 is a 780 nm GaAlAs / GaAs semiconductor laser having an oscillation wavelength of 4, 4 is a collimator lens, and 5 is a collimator lens. Is a condenser lens, 2 is a wavelength conversion element, 3 is an optical waveguide, and 6 is a λ / 2 plate. Reference numeral 7 denotes a fundamental wave emitted from the semiconductor laser, of which 71 is a first pulse light and 72 is a second pulse light. The condenser lens 5 is movable with respect to the optical axis.
And the incident surface of the optical waveguide 3 of the wavelength conversion element 2 are arranged so as to coincide with each other. At this time, the focal point of the second pulse 72 is out of focus.

The wavelength conversion element 2 is an optical waveguide 3 (width × thickness = 1.6 μmx0.32 μm) formed by proton exchange on a LiNbO ₃ substrate of a Z-CUT. The fundamental wave is the minimum TM waveguide mode, and the harmonic is TM radiation. It is a mode of Cherenkov radiation type. 100
Denotes an active layer of the laser 9, and 200 denotes an electric pulse oscillator.

FIG. 2 shows the optical spectrum characteristics of the semiconductor laser (CW oscillation wavelength 780 nm) during pulse operation. The vertical axis is time, and the horizontal axis is the wavelength of light. This is obtained by injecting a bias current 0.5 times the threshold current, a pulse width of 500 ps, and a short current pulse having a repetition frequency of 10 MHz from the electrode 14 and performing gain switching of the semiconductor laser. According to this, the center wavelength of the first pulse of the light pulse is the second wavelength.
It can be seen that the pulse is about 10 nm shorter than the center wavelength of the pulse after the pulse. This is an effect caused by the gain spectrum characteristics of the semiconductor laser, and is described in detail in K. Ketterer, EH Bottche
r, and D.Bimberg Applied Physics Letters Vol53, No.2
3 (2263) 1988. The pulses after the second pulse are caused by relaxation oscillation of the semiconductor laser. FIG. 3A shows an optical pulse waveform observed by an optical oscilloscope. In this figure, the vertical axis is the light intensity,
The horizontal axis is time. The pulse width is less than 40ps and the light output is more than 1W.

The TE mode oscillation light pulse emitted from the semiconductor laser 9 is collimated by the lens 4 as a fundamental wave 7 and λ / 2
After the polarization direction is changed by 90 degrees by the plate 6, the light is condensed by the lens 5 and enters the optical waveguide 3 formed in the wavelength conversion element 2.
At this time, the fundamental velocity of the TM guided mode guided in the optical waveguide 3 and the phase velocity of the second harmonic of the TM radiation mode are equal, and the fundamental wave 7 is the half harmonic of the second harmonic. The light is converted into a light pulse 8 and is Cherenkov radiated.

When the fundamental wave 7 enters the optical waveguide 3, the converging lens 5
Only the first pulse 71 having a short wavelength is incident due to the color convergence (numerical aperture NA = 0.55, focal length f = 4.39 mm). Therefore, only the first pulse of the emitted fundamental wave is wavelength-converted,
The waveform of the second harmonic is a single pulse with very little noise without the second pulse. The waveform of the second harmonic is shown in FIG. 3 (b).

In this embodiment, the focal length of the condenser lens is 0.5 μm / nm with respect to the wavelength of the fundamental wave, and the wavelength of the first pulse is arranged at the position where the wavelength of the first pulse is most efficiently incident on the optical waveguide. Light after the second pulse is not incident on the optical waveguide. As a result, the output waveform of the second harmonic is converted into a single pulse,
The noise ratio was 20 dB or more, far exceeding the conventional 5-10 dB. The improvement of the noise ratio may be different depending on the wavelength dependence of the focal length of the condenser lens.However, if the value of 0.5 μm / nm used in this example is used, it is required for fluorescence lifetime measurement. The design is such that a characteristic of 20 dB or more can be obtained, and at the same time, a light and small device can be realized.

FIG. 4 is a schematic diagram showing an ultrashort pulse generator according to a second embodiment of the present invention. The fundamental wave 7 emitted from the semiconductor laser 9 in basically the same manner as in the first embodiment
Is collimated by the Fresnel lens 44, and the polarization direction is changed by 90 degrees by the λ / 2 plate 6, and then the second Fresnel lens
The light is condensed by 54 and enters the optical waveguide 3.

The Fresnel lenses 44 and 45 are formed by electron beam exposure. When this method is used, the curvature of the Fresnel lens, the pitch of the grating, and the like are controlled by computer control, and therefore, the optimum ratio, the wavelength selectivity, and the like are made with respect to the wavelength of the first pulse of the fundamental wave. Things. There is a difference in focal length of 8 μm between the first pulse and the second pulse of the fundamental wave. This allows
The fundamental wave propagating in the optical waveguide 3 is only the first pulse, the wavelength is converted, and the emitted second harmonic is a single pulse. When a Fresnel lens is used, the effect of the present invention is doubled because the effect of color convergence appears more remarkably than a commonly used spherical glass lens.

Furthermore, when a Fresnel lens is used, the size of the device is 10 mm smaller than when a normal spherical lens is used.

In addition, by forming the first and second Fresnel lenses on a λ / 2 plate, it is possible to make the device small and lightweight.

In this embodiment, the semiconductor laser has a wavelength of 780 nm.
aAsAs / GaAs system was used, but the wavelength was 1.3 μm and 1.55 μm.
m band InGaAsP / InP system or 600nm wavelength AlGaInP /
The same effect can be obtained even in the case of GaAs, and wavelength conversion is possible.

In this embodiment, a normal DH (Double-Hetrostructe) is used.
r) Although a Fabry-Perot laser was used, using an MQW (multi quantum well) laser further widens the wavelength difference between the short wavelength and long wavelength pulse components of the light pulse, so that a further effect is obtained. is there.

In this embodiment, a semiconductor laser that does not control the current injection region is used. However, a part of the resonator is a gain region having an electrode for injecting current into the active layer, and the other part is a saturable absorber. When a semiconductor having a structure that is a region is used, the differential efficiency of the semiconductor laser is increased, so that a higher-output fundamental wave is generated when gain switching is performed.

Further, when the optical waveguide is made a resonator by providing a total reflection film for the fundamental wave on the incident end face and the output end face of the wavelength conversion element, the light density in the optical waveguide further increases, and
The conversion efficiency to harmonics is further improved, and higher output pulses can be realized.

As is clear from the above description, the present invention spatially decomposes a plurality of pulses emitted from a semiconductor laser, makes only the first pulse incident on the optical waveguide, and propagates the light in the optical waveguide. It enables the generation of the second harmonic of a single pulse with a high noise ratio, and prevents the occurrence of optical damage in the optical waveguide by minimizing the average power propagating in the optical waveguide. Is made possible.

Further, by using the condensing grating, a single pulse is made possible, and a further compact and lightweight light source can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an ultrashort optical pulse generator according to a first embodiment of the present invention, FIG. 2 is a diagram showing a wavelength of a semiconductor laser in a pulse operation of the first embodiment, and FIG. FIG. 4 is a pulse waveform diagram of a semiconductor laser beam and a second harmonic, FIG. 4 is a schematic configuration diagram of an ultrashort optical pulse generator according to a second embodiment of the present invention, and FIG. It is a schematic structure figure of a pulse generator. 2 ... wavelength conversion element, 3 ... optical waveguide, 9 ... semiconductor laser, 4 ... collimator lens, 5 ... condensing lens, 7 ... fundamental wave, 71 ... first pulse, 8 ... … Second harmonic, 44, 45 …… Fresnel lens.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/35-1/39 H01S 3/108-3/109 H01S 3/18

Claims (4)

(57) [Claims] 1. A semiconductor laser, a lens system for converting laser light emitted from the semiconductor laser into parallel light, a half-wave plate for rotating the polarization direction of the laser light by 90 degrees, and collecting the parallel light. A condensing lens system, and a waveguide-type light conversion element having a waveguide for converting the wavelength of the laser light, wherein the semiconductor laser is driven by a pulse current, and the laser light emitted from the semiconductor laser is a pulse light. Since only the first pulse of the relaxation oscillation of the semiconductor laser of the pulsed light is incident on the waveguide-type light conversion element having the waveguide, the laser light is focused in the focal length direction in the condenser lens system. An ultrashort optical pulse generator, wherein is capable of wavelength resolution. 2. A lens system for converting a semiconductor laser and laser light emitted from the semiconductor laser into parallel light, a half-wave plate for rotating the polarization direction of the laser light by 90 degrees, and a condensing lens for condensing the parallel light. A system and a waveguide type light conversion element having a waveguide for converting the wavelength of the laser light, the semiconductor laser is driven by a pulse current, and the laser light emitted from the semiconductor laser is a pulse light, Since only the first pulse of the relaxation oscillation of the semiconductor laser of the pulsed light is incident on the waveguide-type light conversion element having the waveguide, the waveguide-type light conversion element includes a first end face of the element. An ultrashort optical pulse generator, comprising: an incident surface for laser light; and a condensing grating provided between the incident surface and a portion that performs the wavelength conversion. 3. A resonator type optical wavelength conversion element, wherein a total reflection film for a wavelength of a fundamental wave is provided on an input end face and an output end face of an optical waveguide formed on the wavelength conversion element. 3. The ultrashort optical pulse generator according to claim 1 or 2. 4. A semiconductor laser, comprising: a resonator including at least an active layer and a confinement layer, wherein a part or all of the resonator is a gain region having an electrode for injecting a current into the active layer; 3. The ultrashort optical pulse generator according to claim 1, wherein the other part of the resonator is a saturable absorption region.