JP3419794B2 - Mode-locked laser and mode-locked optical pulse train generation method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for generating a mode-locked optical pulse train and a mode-locked laser device for generating a mode-locked optical pulse train. In particular, the present invention relates to a picosecond mode-locked laser useful as a master oscillator in optical communication and a light source for spectroscopic experiments.

[0002]

2. Description of the Related Art A mode-locked optical pulse train generation method and a mode-locked laser are very often used for scientific experiments and physical property evaluations that require ultrashort pulsed light having a picosecond to femtosecond time width and highly repetitive optical pulse trains. It is considered to be the most stable and lowest cost method at present. This mode-locking method is broadly classified into a forced mode-locking method and a passive mode-locking method. Specific examples and characteristics of each method are described in "Ultrafast optical technology" (edited by Tatsuo Yajima, published by Maruzen).
See pages 17-197 for details. Both methods are basically characterized in that a modulator for modulating the light intensity or phase or a nonlinear medium is placed in an optical resonator containing an optical amplification medium. Recently, however, a new mode-locking method has been devised by coupling a resonator with an almost equal resonator length to the outside of an optical resonator containing an optical amplification medium, and placing a nonlinear medium inside the resonator, and research has been actively conducted. Has been done. As a typical example, a house (HAH
aus), Eppen, et al., "Optics Letter".
ers), Vol. 14, 1989, p. 48-50. They construct a Fabry-Perot interferometer with a cavity length almost equal to the laser cavity through the output mirror of the laser cavity outside the cavity of the color center laser, and insert an optical fiber into this interferometer. Form a composite resonator laser coupled with a non-linear Fabry-Perot interferometer. An optical pulse is injected into the external resonator in synchronism with the circulation of the optical pulse in the laser resonator, and this optical pulse changes in phase depending on the optical intensity due to the nonlinear substance inserted in the external resonator ( Self-phase modulation). This phase change is largest near the peak of the pulse and small at the tail of the pulse. When the resonator lengths of the external resonator and the laser resonator are made equal, the pulse circulating in the laser resonator and the pulse circulating in the external resonator overlap and interfere on the mirror shared by both central resonators. At this time, if the length of the optical fiber is properly selected, the light waves interfere and cancel each other in the opposite phase in the vicinity of the skirt of the optical pulse, and in the vicinity of the peak, they interfere and strengthen each other in the same phase. It is described that this results in sharpening of the pulse. In fact, they confirmed that this method resulted in an optical pulse width of 127 femtoseconds, which was 23 picoseconds in normal mode locking.

Not only when a non-linear substance that changes the phase of transmitted light according to the light intensity as described above is placed in the external resonator, but also according to the light intensity such as a saturable absorber, the transmitted light is changed. It has been reported that mode-locking occurs even when a non-linear material that changes the intensity of is placed. Quantum well nonlinear mirror whose reflectance changes with light intensity as a mirror constituting an external cavity by U. Keller et al. In 1990, Volume 15, pp. 1377 to 1379 of "Optics Letters". Of passive mode-locking in titanium / sapphire lasers is also described in "Optics Letter".
ERS), Vol. 16, 1991, pp. 384-386, by passively locking the KCl color center laser by placing a semiconductor laser amplifier with current injection in the external cavity according to Grant Grant et al. It has been reported.

The above-mentioned new mode-locking method is an APM
(Additive PulseMode-locki
ng) or CCM (Coupled Cavity)
It is called Mode-locking).

[0005]

The mode locking method as described above has been established and is widely used. However, these mode-locking methods use elements that are extremely difficult to handle and expensive elements, so they are not easy and cost performance is poor.

Further, in recent years, there is a desire to use these mode-locked lasers as master oscillators for optical communication, but in general, these mode-locked lasers are large in size, and it is difficult to make the apparatus compact, and the repetition frequency is high. Is as low as about 100 MHz.

SUMMARY OF THE INVENTION An object of the present invention is to provide a mode-locked laser and a mode-locked optical pulse train generation method which eliminates the above-mentioned drawbacks of the conventional mode-locked method, is excellent in cost performance, and is easy to handle.

[0008]

The first aspect of the present invention
Akira states that a cavity is formed in the layer direction of the active layer, and in an edge-emitting type semiconductor laser that extracts light in this direction, two distributed optical cavity structures having the same equivalent cavity length are formed by three distributed Bragg reflection regions. A mode-locking semiconductor laser having an optical amplification medium and a saturable absorption medium in each of the two optical resonators.

According to a second aspect of the present invention, in a surface-emitting type semiconductor laser in which a resonator is formed in a direction perpendicular to a substrate and light is extracted in this direction, two couplings having an equivalent resonator length are formed by three distributed Bragg reflection regions. Has an optical resonator structure
The mode-locked semiconductor laser is characterized in that the two optical resonators each include an optical amplification medium and a saturable absorption medium.

According to a third aspect of the present invention, the injection current to at least one of the optical amplification medium and the saturable absorption medium of the mode-locked laser is modulated in synchronization with the circulation time of light inside the optical resonator of the mode-locked laser. A method for generating a mode-locked optical pulse train characterized by the above.

According to a fourth aspect of the present invention, one of the two pairs of reflecting mirrors constituting the optical resonator of the laser device includes a saturable absorber inside and has a desired internal optical orbit time. A mode-locked laser characterized by being an etalon composed of parallel plane mirrors having a gap sufficiently shorter than the optical pulse time width.

[0012]

The operation principle of the APM or CCM will be described in more detail with reference to FIG. The configuration of the APM laser is modeled as shown in FIG. The laser resonator 44 is composed of a mirror 41 and a mirror 42, and the external resonator 45 is composed of a mirror 42 and a mirror 43. Four
6 is an optical amplification medium, and 47 is a nonlinear medium. Mirror 4
1, the amplitude reflectance and the amplitude transmittance of the mirror 42 and the mirror 43 are r ₁ , r ₂ , r ₃ , t ₁ , t ₂ , and t ₃ , respectively. It is assumed that the nonlinear medium 47 is sufficiently close to the mirror 43, and the amplitude or phase modulation of the optical pulse generated when the optical pulse reciprocates through the nonlinear medium 47 is m (t).

Attention is now focused on the events that occur on the mirror 42. As shown in FIG. 4, assuming that the amplitudes of light transmitted and reflected by the mirror 42 are a ₁ , a ₂ , b ₁ , and b ₂ , the relational expressions connecting these are written as equations (1) and (2). be able to. b ₁ (t) = r ₂ a ₁ (t) + it ₂ a ₂ (t) Formula (1) b ₂ (t) = it ₂ a ₁ (t) + r ₂ a ₂ (t) Formula (2) It is assumed that the mirror 42 has no loss, and r ₂ ² + t ₂
It is assumed that ² = 1.

The amplitude swing a ₂ (t) at a certain time t is t
Modulation m ^' (t-1 / c) that the optical pulse receives by propagating the optical pulse through the nonlinear medium before the amplitude b ₂ (t-21 / c) one cycle before and 1 / c before t
It is considered to be decided by the equation, and can be written as equation (3). a ₂ (t) = r ₃ b ₂ (t−21 / c) m ^′ (t−1 / c) exp (−iφ) Formula (3) where l is the resonator optical length and c is the speed of light. . exp
(-Iφ) is a component that takes into account the phase difference caused by the difference in resonator length between the laser resonator and the external resonator, and φ = 0 when both resonator length differences are exactly equal. Hereinafter, in order to simplify the notation, it is expressed as m (t) = m ′ (t−1 / c).
That is, m (t) is obtained by shifting the time axis of m '(t) by 1 / c.

In the steady state, the following equation holds. _{a 2 (t-2l / c} ) = a 2 (t) Equation _{(4) b 2 (t-} 2l / c) = b 2 (t) Equation (5) In this case, equation (3) is as follows Can be written on. a ₂ (t) = r ₃ b ₂ (t) m (t) exp (−iφ) Equation (6) When viewed from the laser resonator, if the entire external resonator is regarded as a kind of mirror, its amplitude reflectance is Formulas (1) and (2),
From (6), we can write as in equation (7).

[0016]

[Equation 1]

Expression (7) has the same form as the expression of the reflectance of a nonlinear etalon containing a nonlinear substance inside. Now, especially considering that the nonlinear substance is a saturable absorber, and m (t) represents amplitude modulation, m (t) is 0 ≦ m.
It is a real number that satisfies (t) ≦ 1. When the laser cavity and the external cavity have the same length (φ = 0), the amplitude reflectance r (t) is plotted as a function of m (t) with r ₂ <r ₃ as shown in FIG.
become that way. From this figure, if m (t) is r ₂ / r ₃
If it changes from 1 to 1, r (t) is 0 to (r ₂ −
_{r 3) / (1-r} 2 r 3) until it is found to vary.
If this characteristic is properly used, it is possible to amplify the change in m (t) and reflect it in the reflectance r (t) by properly selecting r ₂ and r ₃ . From the above description, it is understood that in the APM, the external resonator functions to amplify the optical response (optical nonlinearity) of the nonlinear substance contained inside.

FIG. 6B shows a saturable absorber having optical power transmission characteristics | m | ² as shown in FIG. 6A.
An example of calculation of the power reflectivity | r | ² for continuous light when placed in an external resonator with ₂ = √0.5 and r ₃ = √0.99 is shown. I _in is the incident light power and I _s is the saturated light intensity. It is clear from the figure that the external resonator amplifies the nonlinear response of the nonlinear material. This saturable absorber has a light transmittance of 0.7 when it is unsaturated and a light transmittance of 1 when it is saturated. In this case, if r ₂ and r ₃ are selected as described above, the reflectance of the external resonator is Can vary from 0% to about 94%.

In ordinary passive mode-locking, a saturable absorber is inserted in the laser resonator, and mode-locking operation is performed by utilizing the nonlinear response to this light. In general, a large light intensity is required to obtain this nonlinear response, and mode-locked operation can be obtained only with a high-power laser. However, when this APM is used, the optical non-linear response of the non-linear material is emphasized, and therefore mode locking is possible even in a low power laser. The time response characteristic of the nonlinear medium is important for the mode-locked operation, but in general, an optical pulse having a time width sufficiently shorter than the response time of the nonlinear medium has been obtained by mode locking.

As described above, the equation (6) is the same as the equation representing the amplitude reflectance of the nonlinear etalon when the nonlinear medium in which the modulation component for light is represented by m (t) is present. It has the shape of. However, in the case of a non-linear etalon, the exp (-iφ) factor is not the phase difference caused by the difference in resonator length between the laser resonator and the external resonator,
It is the phase difference that occurs when the light makes one round trip in the etalon,
If the mirror spacing of the etalon is 1, then exp (-2il /
It is represented by c). When such a non-linear etalon is used as one of the mirrors constituting the laser resonator, a non-linear external resonator is used when the orbital time of the light in the etalon is sufficiently smaller than the desired optical pulse time width. The same effect as when combined is obtained. This is because, in this case, it is considered that the equations (1) to (6) hold in the nonlinear etalon as in the case of the APM, and thus the equation (7) holds.

This means the following two things. That is, first, in this non-linear external resonator, the mechanism for amplifying the response of the non-linear medium is the effect of multiple reflection and interference of light similar to that of a normal Fabry-Perot etalon.

Second, using a Fabry-Perot etalon containing a nonlinear medium inside one of the mirrors of the laser resonator makes it possible to make the orbital time of the light inside the etalon sufficiently longer than the desired optical pulse time width. If it is short, it is equivalent to coupling a nonlinear external resonator to the laser resonator.

In the APM, it will be explained below whether the non-linear external resonator has the same multiple reflection and interference effect as the ordinary Fabry-Perot etalon, even though the light circulating inside is pulse-like. Like Since the resonator lengths of the laser resonator and the nonlinear external resonator are almost equal,
The light circulation time of both resonators is equal. Therefore, in the steady state, only one light pulse circulates in the external resonator at all times, and when the light pulse arrives at the common mirror, it collides with the light pulse circulating in the laser resonator and transfers energy. Therefore, the optical pulse circulating in the external resonator always receives energy in synchronization with the optical pulse circulating in the laser resonator, and is in a steady state. At this time, it is considered that the optical pulse that circulates in the external resonator is a superposition of the optical pulses that have once circulated once, twice, and three times in the resonator. That is, the interference occurs between the optical pulses that have once made one round, two rounds, three rounds, ... In the resonator.

On the other hand, in the case of the Fabry-Perot etalon, which has a circulation time of 2 l / c sufficiently smaller than the optical pulse width, the multiple reflection and interference effects of light are caused by one light incident on the Fabry-Perot etalon. It is thought to be occurring during the pulse.

From the above description, the nonlinear external resonator coupling of the APM emphasizes the nonlinear optical response of the nonlinear medium to enable mode-locked operation of a low-power laser, and the optical pulse width has a round trip time of 21 / When it is sufficiently larger than c, the Fabry-Perot etalon that includes the nonlinear medium inside emphasizes the optical response (optical nonlinearity) of the nonlinear medium just like the coupling of the nonlinear external resonator, and is extremely effective for mode-locked operation. It has become clear that it behaves favorably.

The first and second inventions of the present invention apply these APM principles to a semiconductor laser to perform passive mode-locking operation, and the third invention additionally provides an optical amplification medium or saturable A fourth aspect of the present invention is a mode in which forced mode-locking is performed by modulating the amount of current injected into an absorbing medium or both, and a fourth invention includes a non-linear medium inside, and the optical circulation time is sufficiently smaller than a desired optical pulse width. A laser device that can obtain mode-locked operation with a simple and low-cost structure by using a Perot etalon as a mirror of a laser resonator.

[0027]

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

FIG. 1 is a structural sectional view showing an embodiment of an edge emitting semiconductor laser according to the present invention . n-type In
A diffraction grating having a period of 2380 A (angstrom) is formed in the distributed Bragg reflection regions 18 a, 18 b and 18 c of the P substrate 11, and undoped InGaAs (wavelength λ _g = 1.3 μm corresponding to the energy gap is used as the light guide layer 14 thereon. , Thickness 0.3 μm), buffer layer 15
As n-type InP (thickness 0.1 μm) as the active layer 12 and the absorption layer 13 as undoped InGaAs (λ _g =
1.53 μm, thickness 0.1 μm), and p-type InP (thickness 0.2 μm) is grown by LPE as the cladding layer 16. Next, the InP clad layer on the distributed Bragg reflection region 18 and I
The nGaAs active layer is selectively removed by etching. Next, a p-type InP clad layer is regrown on the entire surface. After that, the electrode 17 is formed on the active layer 12, the absorption layer 13, and the substrate side. In this way, the element of FIG. 1 is completed. In addition to this structure, although not shown here, it is possible to form a current constriction structure as in a normal semiconductor laser.

The distributed Bragg reflection regions 18 at three locations each have a length of 150 μm, the active region 19 and the absorption region 20 each have a length of 500 μm, and the total length of the device is 145.
It is 0 μm. The equivalent reflectance of the distributed Bragg reflector can be adjusted by the step depth of the diffraction grating, and in this example, the expected reflectance of the region 18a is 80%,
The reflectance of the area 18b is 60%, and the reflectance of the area 18c is 9%.
It is 0%. The light output is extracted from the area 18a.

A mode-locked optical pulse train can be obtained by adjusting the injection current to the active layer 12 and the absorption layer 13. The injection current into the active layer 12 is three times the threshold current value,
When the injection current into the absorption layer 13 was adjusted to about 0.8 times the threshold value, the most stable optical pulse train was obtained, and a continuous mode-locked optical pulse train with a pulse width of about 5 ps and a repetition rate of about 66 GHz was obtained. . At this time, the average optical power is about 50
It was mW.

An optical pulse generating method of the present invention using this semiconductor laser will be described. When the injection current of the laser into the absorption layer was repeatedly modulated at 20% and 66 GHz, the pulse width became narrow to 3 ps. On the other hand, when the injection current into the active layer was repeatedly modulated at 20% and 66 GHz, the pulse width was 2 ps. When both modulations were used together, the pulse width was 1 ps or less. In both cases, it was observed that the pulse width of the optical pulse became narrower as the modulation degree of the injection current was increased.
Even at 0% or more, no further narrowing of the pulse width was observed.

It is also possible to realize the APM laser structure with a surface emitting semiconductor laser. FIG. 2 is a structural sectional view of the surface-emitting mode-locked semiconductor laser of this embodiment. On the n-type GaAs substrate 21, a high resistance layer 26 and a semiconductor multilayer film reflecting mirror 24c composed of 10 pairs of AlAs / GaAlAs layers whose thickness is selected so that the optical length is ¼ of the emission wavelength λ˜880 nm are formed. To do. The expected reflectance is 96%. As the high resistance layer, Fe-doped InP or the like is well known, but an ion-implanted GaAs layer or the like can be used.

A thickness of 10 is formed on the semiconductor multilayer mirror 24c.
0 μm n-type GaAlAs cladding layer 25, 300 μm
Non-doped GaAs absorption layer, 100 μm thick p-type G
The aAlAs clad layer 25a and the high resistance layer 26 are formed. Further, five pairs of AlAs / GaAlAs semiconductor multilayer film reflecting mirrors 24b (reflectance 70%) are formed thereon, and the n-type GaAlAs cladding layer 25b having a thickness of 100 μm is formed again.
300 μm non-doped GaAs active layer 22, thickness 10
A 0 μm p-type GaAlAs cladding layer 25b is formed. Finally, 7 pairs of AlAs / GaAlAs semiconductor multilayer film reflecting mirrors 24a (reflectance 90%) are formed. The element is processed into a mesa shape by selective etching as shown in the figure, and electrodes 27a to 27d are provided. In this way, the element of FIG. 2 is completed. A current is injected into the active layer 22 through the multilayer reflecting mirror 24 using the electrodes 27a and 27b, and a current is injected into the absorbing layer 23 through the multilayer reflecting mirror 24 using the electrodes 27c and 27d. The high resistance layer 26 electrically insulates the active layer 22 and the absorption layer 23, so that current injection into both can be controlled independently.

A mode-locked optical pulse train can be obtained by adjusting the injection current to the active layer 22 and the absorption layer 23. The injection current to the active layer 22 is three times the threshold current value,
When the injection current into the absorption layer 23 was adjusted to about 0.8 times the threshold value, the most stable optical pulse train was obtained, and a continuous mode-locked optical pulse train with a pulse width of about 5 ps and a repetition rate of about 80 GHz was obtained. . At this time, the average optical power is about 30.
It was mW.

Further, the modulation current of the injection current to the absorption layer is set to 20.
%, The pulse width was 1 ps or less when repeatedly modulated at 80 GHz.

Next , an embodiment of the present invention will be described with reference to FIG. FIG. 3 is a schematic diagram showing the configuration of the mode-locked laser, where 31 is an output mirror and 33 is an optical amplification medium. 32
Is a non-linear etalon whose structure is shown enlarged in a circle. The laser resonator comprises an output mirror 31 and a non-linear etalon 32. The non-linear etalon 32 is composed of a saturable absorber 34 sandwiched by multilayer film reflecting mirrors 35a and 35b formed on a substrate 36.

In this example, an example using a titanium / sapphire laser as the laser used will be described. The oscillation wavelength of the titanium / sapphire laser is 700 to 900.
nm, and G as a saturable absorber in this wavelength range
aAs can be used. The actual nonlinear etalon is composed of 10 pairs of AlAs / GaAlAs semiconductor multilayer film reflecting mirrors 35b (reflecting on the GaAs substrate 36, the thickness of which is selected to be 1/4 of the laser oscillation wavelength λ and 880 nm). Rate ~ 95%), 100 nm thick GaA
s and 4 pairs of AlAs / GaAlAs semiconductor multilayer film reflectors 35a (50% from reflector) were made by MBE growth. The expected orbital time of the etalon is about 2 ps.

Experiments have confirmed that this simple configuration results in passive mode locking in the titanium / sapphire laser. The obtained optical pulse width is at least 10 ps
And the repetition frequency was determined by the length of the laser resonator and was about 120 MHz in this case. It was found that stable mode-locking was obtained in the wavelength range of 830 to 850 nm, and was obtained at periodic wavelengths every 0.6 nm corresponding to the resonance peak of the etalon.

In the embodiments shown above, an example of a semiconductor laser using InGaAs is used as an example of an edge-emitting mode-locked semiconductor laser, and an example of a semiconductor laser using GaAs is used as an example of a surface-emitting mode-locked semiconductor laser. As an example of a mode-locked laser using a nonlinear etalon, a titanium / sapphire laser is used as a saturable absorber and a GaA is used as a saturable absorber.
Although the example using the s-semiconductor has been described, the material and composition of the element are not limited to those in the above-described embodiments, and other semiconductor materials or dielectric materials may be used. The saturable absorber may be any suitable one for the laser used.

[0040]

As described above, according to the present invention,
A mode-locked laser with excellent cost performance and easy handling can be realized.

[Brief description of drawings]

FIG. 1 is a structural cross-sectional view of a mode-locking edge-emitting semiconductor laser for explaining an embodiment of the present invention.

FIG. 2 is a structural sectional view of a mode-locking surface-emitting type semiconductor laser for explaining an embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a mode-locked laser for explaining an embodiment of the present invention.

FIG. 4 is a laser model diagram for explaining the principle of an APM or CCM laser.

FIG. 5 is a diagram showing a relationship between an amplitude reflectance r and an amplitude modulation rate m.

FIG. 6 is a diagram showing the relationship between incident light power (I _in / I _s ), optical power transmission characteristics | m | ² and power reflectance | r | ² .

[Explanation of symbols]

11 board 12 Active layer 13 Absorption layer 14 Guide layer 15 Buffer layer 16 Clad layer 17 electrodes 18a distributed Bragg reflection region 18b distributed Bragg reflection region 18c distributed Bragg reflection area 19 Active area 20 Absorption area 21 board 22 Active layer 23 Absorption layer 24a multilayer film reflector 24b Multilayer film mirror 24c multilayer mirror 25a clad layer 25b clad layer 26 High resistance layer 27a electrode 27b electrode 27c electrode 27d electrode 31 output mirror 32 Non-linear etalon 33 Optical amplification medium 34 Saturable absorber 35a multilayer mirror 35b Multi-layer film mirror 36 board 41 Mirror (M1) 42 Mirror (M2) 43 Mirror (M3) 44 laser resonator 45 External resonator 46 Optical amplification medium 47 Non-linear medium

Claims (5)

(57) [Claims] 1. An edge-emitting type semiconductor laser in which a resonator is formed in a layer direction of an active layer, and APM mode locking is performed to extract light in this direction, and an equivalent resonator length is made equal by three distributed Bragg reflection regions. A mode-locked laser having two series-coupled resonator structures, one of which contains an optical amplification medium and the other of which contains a saturable absorption medium. 2. A resonator is formed in a direction perpendicular to the substrate,
A surface-emitting type semiconductor laser that performs APM mode locking to extract light in this direction, and has a resonator structure in which two distributed Bragg reflection regions have an equivalent resonator length and are coupled in series. A mode-locked semiconductor laser, characterized in that one contains an optical amplification medium and the other contains a saturable absorption medium. 3. The Bragg reflection regions are all equal in length, and the resonator containing the first-stage optical amplification medium and the resonator containing the first-stage saturable absorption medium are the same length. Or the mode-locked laser described in 2. 4. The injection current to at least one of the optical amplification medium and the saturable absorption medium of the mode-locked semiconductor laser according to claim 1 is synchronized with the circulation time of light inside the optical resonator of the mode-locked laser. A mode-locked optical pulse train generation method characterized by modulating. 5. One of the reflectors of the reflector <br/> constituting an optical resonator of the laser device comprises a saturable absorber medium therein, and the interior of the optical circuit time desired optical pulse time width A mode-locked laser which is an etalon composed of parallel plane mirrors having a gap sufficiently shorter than that.