JP2510348B2 - Laser device for short pulse generation - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor laser device that generates a short pulse in communication, information processing, and high-speed measurement.

Here, the short pulse is a pulse having a pulse width of 10 ^-12 seconds or less.

(Problems to be Solved by Conventional Techniques and Inventions) Optical pulse generation using a conventional semiconductor laser uses a semiconductor laser alone (a), a semiconductor in which an antireflection coating is applied to an end face in an external resonator. It can be divided into one (b) containing a laser as an amplification medium.

(B) In this conventional method, a semiconductor laser is excited by a current pulse. When the current amplitude of the pulse becomes larger than the oscillation threshold current, light is emitted, and when the current amplitude becomes small, the light emission is stopped to form an optical pulse. However, since the light pulse width is determined by the current pulse width, only pulses in the 10 ^-11 second region are obtained. In addition, the electrode of the semiconductor laser is divided into an amplifying medium portion that forms a population inversion and a saturable absorbing medium portion that does not form a population inversion. The amplifying medium portion is continuously injected and excited, and the passive mode is generated by the action of the saturable absorbing medium. There have also been attempts to generate pulses by applying synchronization (see Note 1).

Note 1 Passive mode synchronization As shown in Fig. 5, a pulse occurs when the gain exceeds the loss. The saturable absorbing medium has such a loss curve with respect to the gain curve. In addition, such behavior is said to have taken passive mode synchronization.

However, in such a system, the repetition time of the optical pulse (about 10 ^-11 seconds) is shorter than the recovery time of the gain and absorption, so that there is a drawback that mode locking is difficult to take.

(B) In this conventional method, a semiconductor laser having a single-sided antireflection film and a single-sided high-reflection film is used as a single end reflector (see FIG. 4), and a double-sided antireflection film is applied. There was one in which a resonator was composed of a semiconductor laser and two other mirrors. This was excited by a current pulse synchronized with the round-trip time of the light of the resonator to generate a short pulse, or a saturable absorption band was provided in the semiconductor laser to generate a short pulse by passive mode locking. . The pulse widths are about 10 ^-11 seconds and 10 ^-12 seconds, respectively. The average light output is several mW. As described above, the reason why the average light output is smaller and the pulse width is not narrower than that of the dye laser and the solid-state laser is mainly due to the following reasons.

(1) Since the spot size of the oscillation mode of the semiconductor laser is small on the end face, the end face cannot withstand high output.

(2) It is difficult to coat the antireflection film due to structural factors, and it operates as a composite resonator, and thus it is difficult to obtain a stable pulse.

(3) The spot size of the oscillation mode is small, and the coupling efficiency between the mode of the external resonator and the internal mode of the semiconductor laser is poor.

(4) Since the active region is distributed as long as several hundreds of microns, the mode-locking efficiency decreases due to the active Fabry-Perot effect of the semiconductor laser amplification medium itself.

(Means for Solving the Problems) In order to solve the above problems, according to the present invention, a surface emitting semiconductor laser that emits light in a crystal growth direction is arranged as an amplification medium in an external resonator (see FIG. 6), and a mode is set. By synchronizing, short pulse (width is high) of high average output (10mW or more)
The reasons why the surface emitting semiconductor laser, which is capable of generating 10 ⁻¹² seconds or less), can solve the above problems are listed below.

(1) Since the surface emitting semiconductor has a large light emitting area,
A large output can be obtained even with the same output density as that of a normal edge emitting semiconductor laser.

(2) Since the area of the light emitting region is large, the antireflection film coating is easy. Also, since diagonal polishing is easy,
The operation as a composite resonator can be further suppressed.

(3) Since the area of the light emitting region is large, the coupling efficiency with the external resonator mode is high.

(4) Since the width of the active region is sufficiently short, the active Fabry-Perot effect of the semiconductor laser amplification medium itself can be suppressed.

The present invention has the following features in addition to the features described above.

(1) Since the semiconductor laser medium is excited by the pump light spatially having a Gaussian distribution, the central part of the pump light is
It acts as an amplifying medium and acts as a saturable absorbing medium in the peripheral portion of the pump light. For this reason, passive mode-locking is applied without inserting another saturable absorbing medium in the external resonator, so that the system is simplified.

(2) The emission wavelength is close to the absorption edge wavelength of the semiconductor substrate and can be set in the transparent region. Also, unlike ordinary semiconductor lasers, the optical peak power in the resonator is high, so the self-phase modulation is large due to the third-order nonlinearity of the semiconductor substrate, and pulse compression can be performed by the soliton effect (see Note 2).

Note 2 Soliton effect Normally, when an optical pulse propagates through a third-order nonlinear medium (here, a semiconductor), as shown in Fig. 8 (a), due to the third-order nonlinearity, the tip part has a long wavelength and the tail part has a long wavelength. It is modulated to have a short wavelength. Since the long wavelength portion travels slower than the short wavelength portion, stable propagation cannot be obtained. However, if the pulse width becomes narrow enough,
A pulse having a stable propagation characteristic, that is, a soliton as shown in FIG. 8B is formed by the third-order nonlinearity and the negative dispersion proportional to the nonlinearity. Moreover, such a process is called the soliton effect.

(3) The emission wavelength of the semiconductor laser can be made variable by changing the composition of the semiconductor mixed crystal to shift the band gap. Therefore, if the composition of the surface emitting semiconductor laser medium is changed within the surface, the oscillation wavelength can be changed over a wide range by sliding the light emitting region (see FIG. 7).

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

Example 1 FIG. 1 (a) shows a first example of the present invention, in which a mirror for reflecting laser light and a reflecting mirror in which gold is coated on one side of a surface emitting semiconductor laser medium are used as terminal reflectors for resonance. It is a block diagram of the laser apparatus which comprised the container.

In FIG. 1A, reference numeral 1 is an optical isolator, which prevents the reflected pump light from entering the pump light laser device. Reference numeral 2 is a pump light, which is a pulsed light due to mode locking. A dichroic mirror 3 reflects the pump light and transmits the signal light. Reference numeral 4 denotes a terminal reflector that transmits pump light and reflects laser light. Reference numeral 5 denotes an amplification medium semiconductor having an antireflection film on one surface and gold coating on one surface. Form an external resonator. 6
Is a lens, which collects light on the amplification medium 5. 7
Is the output light.

In the above system, mode synchronization as a pump generator
Nd Yag laser (emission wavelength 1.32 μm), multiple quantum well InGaAs semiconductor with absorption edge wavelength at 1.55 μm as a semiconductor laser medium (well layer thickness 15 nm, total well layer number 200, InP
The results obtained with a barrier layer thickness of 7.5 nm) are as follows.

(1) 200mW average output (> several mW), ie about 20%
Quantum efficiency (see Fig. 9) (2) Stable mode-locked pulse (100MHz repetition) (see Fig. 10) Fig. 10 shows that the output pulse light waveform is stable. However, the output pulse light width is actually about 700 fs, but it is apparently about 100 ps due to the performance limit of the light receiving element.
Has become.

(3) Fluorescence spectrum width of 30 nm (see Fig. 11) Having a fluorescence spectrum width of 30 nm means 10 ^-13
It shows the possibility of obtaining a pulse in the region.

Although an antireflection film is applied to one end surface of the semiconductor 5 for amplification medium, it is further polished obliquely to form an inclined end surface, so that the Fabry-Perot is defined by the cavity length of the semiconductor 5 for amplification medium. The mode can be suppressed completely with high accuracy. Needless to say, the structure in which the end face having the antireflection film is an inclined end face is also effective in other embodiments described below.

Embodiment 2 FIG. 1 (b) shows a second embodiment of the present invention in which a surface emitting semiconductor is used as a laser medium, two mirrors for reflecting laser light are used, and a resonator is used as a terminal reflector. It is a block diagram of the laser device.

In FIG. 1B, a terminal reflector 8 is a terminal reflector that transmits pump light and reflects laser light. The terminal reflector 8 forms an external resonator in combination with the terminal reflector 4. . Reference numeral 9 denotes a semiconductor for an amplification medium whose both surfaces are coated with an antireflection film. Lenses 10 and 11 focus light on the amplification medium. Reference numeral 12 is pump transmitted light.

Embodiment 3 FIG. 2 (a) shows a third embodiment of the present invention, in which a mirror for reflecting laser light and a reflecting mirror in which gold is coated on one side of a surface emitting semiconductor laser medium are used as a terminal reflector for resonance. FIG. 3 is a configuration diagram of a laser device that configures a container, and another semiconductor as a saturable absorption medium is arranged in the optical axis thereof.

In FIG. 2 (a), 13 transmits the pump light,
It is a mirror that reflects the laser light so that the pump light does not hit the saturable absorption medium 15. Reference numeral 14 denotes a terminal reflector, which forms a pair with the gold-coated surface of the semiconductor 5 for an amplification medium to form an external resonator. 16 and
Reference numeral 17 denotes a lens, which collects light on the saturable absorbing medium 15 which has a function of shortening the pulse width.

Example 4 FIG. 2 (b) shows a fourth example of the present invention in which a surface emitting semiconductor is used as a laser medium, two mirrors for reflecting laser light are used, and a resonator is configured as a terminal reflector. FIG. 3 is a configuration diagram of a laser device in which another semiconductor as a saturable absorbing medium is arranged in its optical axis.

In FIG. 2 (b), 4 and 14 are terminal reflectors, which form external resonators. Curved mirrors 18 and 19 focus the light on the amplification medium 9. Numeral 19 is a mirror that transmits the pump light and reflects the laser light so that the pump light does not hit the saturable absorption medium 15. Curve mirrors 20 and 21 focus light on the saturable absorbing medium 15.

Fifth Embodiment FIG. 3 (a) shows a fifth embodiment of the present invention, in which a mirror for reflecting laser light and a reflecting mirror in which gold is coated on one side of a surface emitting semiconductor laser medium are used as terminal reflectors for resonance. FIG. 3 is a configuration diagram of a laser device that constitutes a container, and another semiconductor as a saturable absorption medium and a prism for generating a soliton effect are arranged in the optical axis thereof.

In FIG. 3 (a), 14 is a terminal reflector, which forms an external resonator in combination with the gold-coated surface of the semiconductor 5 for amplification medium. Reference numerals 22 and 23 denote linear prisms, which provide dispersion so as to compensate for the phase change of light generated by the semiconductor 5 for amplification medium and the saturable absorption medium 15.

Embodiment 6 FIG. 3 (b) shows a sixth embodiment of the present invention in which a surface emitting semiconductor is used as a laser medium, two mirrors for reflecting laser light are used, and a resonator is used as a terminal reflector. FIG. 3 is a configuration diagram of a laser device in which another semiconductor as a saturable absorbing medium and a prism for producing a soliton effect are arranged in the optical axis.

In FIG. 3B, the end reflector 4 and the end reflector 14
Form an external resonator.

(Effects of the Invention) As described above, by using the laser device for generating a short pulse of the present invention, it is possible to apply to communication, information processing, and high-speed precision measurement in the femtosecond region of 10 ^-12 seconds or less, and wavelength Variable, high power light pulses can be generated using a semiconductor laser.

[Brief description of drawings]

FIG. 1 (a) is a configuration diagram of a laser device in which a resonator is configured with a mirror that reflects laser light and a reflecting mirror having one surface of a surface emitting semiconductor laser medium coated with gold as a terminal reflector, and FIG. ) Is a configuration diagram of a laser device in which a surface emitting semiconductor is used as a laser medium, two mirrors that reflect laser light are configured, and a resonator is configured as a terminal reflector, and FIG. A laser device in which a mirror and a reflecting mirror coated with gold on one side of a surface-emitting semiconductor laser medium are used as terminal reflectors to form a resonator, and another semiconductor serving as a saturable absorbing medium is arranged in the optical axis of the resonator. FIG. 2 (b) shows that a surface-emitting semiconductor is used as a laser medium, two mirrors that reflect laser light are used as a terminal reflector to form a resonator, and a saturable absorption medium is formed in the optical axis thereof. Another semiconductor placed FIG. 3 (a) is a configuration diagram of a laser device in which a resonator is configured by using a mirror that reflects laser light and a reflecting mirror that is gold-coated on one side of a surface-emitting semiconductor laser medium as a terminal reflector. FIG. 3B is a configuration diagram of a laser device in which another semiconductor as a saturable absorption medium and a prism for generating a soliton effect are arranged in the axis, and FIG. A laser device in which two mirrors that reflect each other constitute a resonator as a terminal reflector, and another semiconductor as a saturable absorbing medium and a prism for producing a soliton effect are arranged in the optical axis of the resonator Configuration diagram, FIG. 4 is a configuration diagram of a conventional mode-locked laser device using an edge-emitting semiconductor laser having an external resonator, and FIG. 5 is a diagram illustrating a mechanism of pulse generation by passive mode-locking, FIG. 6 is a block diagram of a mode-locked laser device using a surface-emitting semiconductor laser having an external resonator according to the present invention, and FIG. 7 shows a wavelength tunable by changing the composition of the surface-emitting semiconductor mixed crystal. FIG. 8 (a) shows a normal pulse propagating in a nonlinear medium, FIG. 8 (b) shows a soliton pulse propagating in a nonlinear medium, and FIG. 9 shows the present invention. The figure which shows the relationship between the laser output power of a laser apparatus and the laser input power for optical pumps, FIG. 10 is a figure which shows the laser pulse waveform of the laser apparatus of this invention, FIG. 11 is the laser used in Example 1 of this invention. It is a figure which shows the fluorescence spectrum of a medium. 1 ... Optical isolator, 2 ... Pump light 3 ... Dichroic mirror 4 ... Terminal reflector 5 ... Antireflection film on one surface, gold-coated semiconductor for amplification medium on one surface 6 ... Lens, 7 ... Output Light 8 …… Terminal reflector 9 …… Semiconductor for amplification medium coated with antireflection coating on both sides 10 …… Lens, 11 …… Lens 12 …… Pump transmitted light 13 …… Dichroic mirror 14 …… Terminal reflector, 15 …… Saturable absorption medium 16 …… Lens, 17 …… Lens 18 …… Dichroic curve mirror 19 …… Curve mirror, 20 …… Curve mirror 21 …… Curve mirror, 22 …… Linear prism 23 …… Linear prism, 24 ...... Semiconductor laser 25 …… High-reflection coating, 26 …… Active layer 27 …… Non-reflection coating, 28 …… Lens 29 …… Mirror

 ─────────────────────────────────────────────────── --- Continuation of the front page (72) Inventor, Jiang Wen-Jin, No. 25 Laoximen Cao Street, Nanshi District, Shanghai City, People's Republic of China (56) Reference JP-A-4-56293 (JP, A)

Claims (4)

(57) [Claims] 1. A resonator comprising a mirror reflecting laser light and a reflecting mirror coated on one side of a semiconductor amplifying medium as terminal reflectors, wherein the semiconductor amplifying medium is a surface emitting amplifying medium. The semiconductor amplification medium has a non-reflection coating on the surface opposite to the surface coated with the reflection mirror, and the pump light is a repetitive pulse having the same cycle as the round trip time of the light of the resonator. Laser device for short pulse generation. 2. A semiconductor amplification medium in which a mirror for reflecting laser light and another high-reflection mirror are respectively configured as terminal reflectors, and both surfaces of which act as surface-emitting laser medium are antireflection coated. Is disposed in the resonator, and the pump light is a repetitive pulse having the same cycle as the round-trip time of the light of the resonator. 3. The semiconductor device according to claim 1, wherein another semiconductor saturable absorption medium having antireflection coating on both sides is arranged in the resonator of the laser device for generating short pulses. A mirror that transmits the pump light and reflects the laser light is arranged in the resonator so that the pump light does not enter the saturated absorption medium, and the wavelength at the absorption edge of the semiconductor saturable absorption medium is the laser. A laser device for generating a short pulse, which has a wavelength substantially equal to that of light. 4. A negative pulse compensating for self-phase modulation caused by third-order nonlinearity in the semiconductor amplifying medium or the semiconductor saturable absorbing medium in the resonator of the short pulse generating laser device according to claim 3. A short pulse generating laser device, wherein a dispersion medium having frequency dispersion is arranged, and the dispersion medium is arranged so that pump light does not enter the dispersion medium.