JPH11121873A - Mode-locked laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a mode-locked laser which is capable of oscillating pulses narrower in width and higher in pulse repetition frequency than those that a conventional mode-locked laser is able to oscillate. SOLUTION: Amplifying regions 67 whose peak positions of gain bands are different from each other are arranged between mirrors 65 separating spatially from each other. Then, a longitudinal mode 69 is spatially branched by a branching filter 66 into modes 69 of frequencies f1 to f5 , and the modes 69 of frequencies f1 to f5 are each amplified by the light amplifying mediums 67. Voltage pulses of frequency equal to integer times or 1/integer as high as a frequency interval Δf (=fk+1 -fk , k=1 to N-1) are applied to the light amplifying mediums 67.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a long-distance and large-capacity optical communication system comprising a time division multiplex communication system, a wavelength division multiplex communication system, or a hybrid communication system of a time division multiplex communication system and a wavelength division multiplex communication system. Ultra-short optical pulse generator indispensable for use in ultra-high speed optical measurement.

[0002]

2. Description of the Related Art FIG. 7 is a sectional view showing the configuration of a typical conventional active mode-locked semiconductor laser. FIGS. 8A and 8B are diagrams for explaining the basic operation of the mode-locked laser. FIG. 8A shows a pulse train, and FIG. 8B shows a relationship between a gain spectrum and a longitudinal mode. In this semiconductor laser, a clad layer 52, an active layer 53, and a clad layer 52 are sequentially laminated on a crystal substrate 51, and an n-type electrode 55 is formed on the lower surface of the crystal substrate 51, and a p-type electrode 54 is formed on the upper clad layer 52. A voltage pulse power supply 56 is connected between the electrodes 54 and 55.

When the semiconductor laser is subjected to gain modulation at a frequency equal to an integral multiple or a fraction of the longitudinal mode interval determined by the laser cavity length from a pulse power supply 56, the longitudinal spectrum existing in the gain spectrum is applied. The phases are synchronized between the modes 59, and a pulse train 57 is generated. The pulse width Δt obtained from the mode-locked laser is the reciprocal of the gain bandwidth Δν _g (Δt = 1 / Δν _g ), and the repetition frequency f of the pulse train is given by the longitudinal mode interval Δν constituting the pulse (f = Δν). .

[0004]

In order to generate a pulse train of short pulses with high repetition, the gain bandwidth Δν _g
Need to be as large as possible. However, in an external cavity mode-locked laser or an integrated mode-locked laser using a conventional semiconductor material as an active medium, the gain bandwidth of the semiconductor medium is at most about several terahertz (terahertz: THz = 1 × 10 ¹² (Hz). )), The light pulse width that can be generated is about 200 fs (fs: femtosecond = 1).
× 10 ^-15 (sec)), and the repetition frequency is limited to about 5 THz.

The present invention has been made in view of the current situation of such a semiconductor mode-locked laser, and has a semiconductor mode-locked laser capable of realizing a pulse width and a repetition frequency exceeding the limits of a conventional semiconductor mode-locked laser. The purpose is to provide.

[0006]

According to the present invention, an amplification region for amplifying each longitudinal mode is spatially separated, and each longitudinal mode is amplified by an optical amplifying medium suitable for the mode, so that an effective gain bandwidth is obtained. The above-mentioned object is achieved by expanding. Hereinafter, FIG. 9 illustrating the amplifying operation of the conventional mode-locked laser and FIG. 10 illustrating the amplifying operation of the mode-locked laser according to the present invention.
The principle of the present invention will be described in detail with reference to FIG. 9A and 10A are diagrams illustrating a basic configuration of a laser, and FIG. 9B is a diagram illustrating a relationship between a gain spectrum and a longitudinal mode.

As shown in FIG. 9, in a conventional mode-locked laser, an optical amplifying medium 62 is disposed between mirrors 60, and a frequency interval Δf (= f) of a longitudinal mode 61 from a voltage pulse power supply 63 is applied to the optical amplifying medium. _{k + 1} −f _k , k = _{1 to} N−
Laser oscillation is performed by applying a voltage pulse having a frequency equal to an integral multiple of 1) or a fraction of 1). That is, each longitudinal mode 61 constituting a pulse is amplified by the same optical amplification medium 62. Therefore, the pulse width and the repetition frequency are equal to the gain bandwidth Δ of the gain spectrum 64 of the one optical amplification medium 62.
Limited by ν _g .

On the other hand, in the present invention, as shown in FIG.
Amplifying regions 67 having different gain band peak positions are spatially separated between mirrors 65. Next, spatially demultiplexes the longitudinal mode 69 in the demultiplexer 66, the frequency f ₁ ~f ₅
Each mode 69 is amplified by each optical amplifying medium 67. That is, FIG. 10 as in (B), the mode of the frequency f ₁ is amplified by the optical amplification medium 1 having a peak of a gain band in the vicinity of frequency f _1, the mode of the frequency f ₂ is the frequency f ₂
Amplified by the optical amplifying medium 2 having a gain band peak in the vicinity, and each of the modes having the frequencies f ₃ , f ₄ , and f ₅ similarly has light having a gain band peak in the vicinity of the frequencies f ₃ , f ₄ , and f _5. Amplify with amplification media 3, 4, and 5. The optical amplification medium 67 has a frequency interval Δf of a longitudinal mode 69 from a voltage pulse power supply 68.
A voltage pulse having a frequency equal to an integral multiple of (= f _{k + 1} −f _k , k = 1 to N−1) or a fraction thereof is applied.

In this manner, the longitudinal modes are spatially separated and demultiplexed, and the demultiplexed modes are amplified by different optical amplifying media according to their frequencies. Even if the gain bandwidth is limited, the effective gain bandwidth is the gain bandwidth Δν of each optical amplification medium.
_Since it is determined by the sum of _g , it is possible to obtain a gain bandwidth Δν ′ _g that greatly exceeds the limitation of each gain bandwidth (Δν ′ _g
≫Δν _g ). Therefore, it is possible to generate a pulse train having a pulse width and a repetition frequency exceeding the limits of the semiconductor.

That is, the mode-locked laser according to the present invention comprises:
An optical element that splits an incident optical pulse into a plurality of frequency components and spatially separates and outputs the frequency components, and a gain for individually amplifying each frequency component that is spatially separated and output from the optical element. It is characterized by including a plurality of optical amplification media having different peak positions in the band. The plurality of frequency components have equal frequency intervals, and the plurality of optical amplification media individually amplify the frequency components incident on each optical amplification medium. An optical element that splits an incident light pulse into a plurality of frequency components and spatially separates and outputs the separated light components can be realized by an optical waveguide grating router (AWG) or an AWG (array waveguide grating). The number of optical amplification media is the same as the number of split frequencies.

Further, the mode-locked laser according to the present invention splits an optical pulse incident on one input optical waveguide into a plurality of frequency components having equally spaced frequency intervals, and converts each frequency component into a plurality of different output optical waveguides. , And a plurality of electrically separated optical amplification media coupled to each output optical waveguide of the optical element. A plurality of optical amplifying media need not be spatially and electrically separated, but if the current flowing through the optical amplifying media is biased by using a common electrode, each optical amplifying medium is electrically isolated. Then, it is preferable to provide a dedicated electrode for each.

An active mode-locked laser can be realized by applying an electric signal or an optical signal having a frequency equal to an integral multiple or a fraction of the frequency interval of the plurality of frequency components to the optical amplification medium. By providing the saturable absorption region, a passively mode-locked laser can be realized. Further, a hybrid mode-locked laser is realized by applying an electric signal or an optical signal having a frequency equal to an integral multiple or a fraction of the frequency interval of the plurality of frequency components to the saturable absorption region and the plurality of optical amplifying media. be able to.

In an optical communication system, optical signals for signal transmission are multiplexed on the time axis, and optical pulses having a pulse width as narrow as possible are transmitted with a narrowed time interval, thereby transmitting as many signals as possible per unit time. Time-division multiplexing communication system for transmission (multiplexing of signals in time space),
A wavelength division multiplexing communication system (multiplexing of signals in frequency space) that transmits as many signals as possible per unit time using a plurality of optical pulses of different wavelengths, and also in space in both time space and frequency space. There is a hybrid communication system of a time division multiplex communication system and a wavelength division multiplex communication system that perform more signal transmission by performing signal multiplexing.

By using the mode-locked laser according to the present invention as a short pulse light source, a time-division multiplexing optical communication system of 1 terabit / sec to 10 terabit / sec class, which greatly exceeds the current transmission speed of 10 gigabit / sec, It is possible to realize a wavelength division multiplexing optical communication system and a hybrid optical communication system in which wavelength division multiplexing and time division multiplexing are mixed.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a mode-locked laser according to the present invention will be described below in detail with reference to the accompanying drawings. <First Embodiment> First, referring to FIGS. 1 and 2, an optical waveguide grating router and the same number of spatial (electrical) as the number of wavelengths (frequency) spatially demultiplexed by the optical waveguide grating router will be described. An example of an active-locked mode-locked laser composed of an optical amplification medium separated in (1) will be described. FIG. 1 is a plan view of this mode-locked semiconductor laser, and FIG. 2 is a cross-sectional view showing a method of forming an amplification region.

N-type InP on the (100) crystal orientation (forbidden bandwidth: λ _g = 0.92 μm, donor concentration: N _D = 2)
A (1 × N) optical waveguide grating router 1 made of InGaAsP is formed on a (× 10 ¹⁸ cm ⁻³ ) substrate (M. Zirngibl et al., IEEE Photon. Technol. Le.
tt., 3 , pp. 812-815 (1991)). The (1 × N) waveguide grating router 1, a light pulse incident from the optical waveguide 2 N pieces of wavelength (frequency) components lambda ₁ to [lambda] _N
(F _{1 to} f _N ) are spatially decomposed into optical waveguides 4, and amplification regions 5 (A _{1 to} A ₁ ) connecting them to the respective optical waveguides 4.
_N ). Therefore, each wavelength component is amplified in a different amplification region 5 (A _{1 to} A _N ). Here, the frequency interval Δf = f _{k + 1} −f _k (k = 1 to N−1) is constant.

A method for forming each amplification region 5 will be described with reference to the cross-sectional view of FIG. First, as shown in FIG. 2A, an n-type In without forming the optical waveguide grating router 1 is used.
Metalorganic vapor phase epitaxy (MOVPE: Metal)
The Organic Vapor Phase Epitaxy) method, an undoped InGaAsP clad layer (bandgap wavelength: λ _g = 1.
(1 μm, thickness: 0.5 μm) 11 is formed. Next, as shown in FIG. 2B, an active layer of the amplification region A _k (k = 1 to N) is formed of InGaAsP (thickness: 0.1 μm) 12. At this time, the portion other than the amplification region _Ak is a mask 71
To prevent crystal growth. In addition, the forbidden bandwidth wavelength of each active layer 12 is set to be longer than λ _{k so} that the wavelength λ _k is efficiently amplified.

As shown in FIG. 2C, each amplification region A
After growing the active layer 12 of _k (k = 1 to N), FIG.
As shown in (D), undoped InGaAs is formed thereon.
P cladding layer (forbidden bandwidth: λ _g = 1.1 μm, thickness: 0.5 μm) 13, p-type InP cap layer (λ _g =
0.92 μm, acceptor concentration: N _A = 5 × 10 ¹⁸ c
m ⁻³ , thickness: 0.1 μm) 14. Next, FIG.
As shown in (E), each amplification region A _k (k = 1 to N) is spatially (electrically) separated by chemical etching, and mirrors 7 and 8 are formed by cleavage. Finally, as shown in FIG. 2F, an n-side electrode 15 and a p-side electrode 16 are formed by depositing AuGeNi / Au and Au / AuZn, respectively.

A voltage pulse power source 6 is connected to the electrodes of each amplification region 5.
As a result, an electric signal having a frequency equal to the frequency interval Δf of the (1 × N) optical waveguide grating router 1 is applied, mode locking is actively performed, and a short optical pulse train 9 is generated. According to this mode-locked semiconductor laser, 10f
It is possible to generate a pulse train having a pulse width of about s to 100 fs.

<Embodiment 2> Next, referring to FIGS. 3 and 4, an optical waveguide grating router and the same number of spatial wavelengths (frequency) as spatially demultiplexed by the optical waveguide grating router will be described. Another example of an active-locked mode-locked laser composed of (electrically) separated optical amplification media will be described. FIG. 3 is a plan view of the mode-locked semiconductor laser, and FIG. 4 is a cross-sectional view for explaining a method of forming an amplification region. In this example, the growth method of the amplification region is different from that of the first embodiment.

N-type InP on the (100) crystal orientation (forbidden bandwidth: λ _g = 0.92 μm, donor concentration: N _D = 2)
(× 10 ¹⁸ cm ⁻³ ) InGaAsP on a substrate (1
× N) of the optical waveguide grating router 17 is formed. The optical waveguide grating router 17 spatially decomposes _N wavelength (frequency) components λ _{1 to} λ _N (f _{1 to} f _N ) of the optical pulse incident from the optical waveguide 18 into an optical waveguide 20, and decomposes each of them. To the amplification region 21 (A _{1 to} A _N ) connected to the optical waveguide 20. Therefore, each wavelength component is amplified in a different amplification region 21 (A _{1 to} A _N ). Here, the frequency interval Δf = f _{K + 1} −f
_K (k = 1 to N-1) is constant.

Referring to FIG. 4, a method for forming each amplification region will be described. First, as shown in FIG. 4A, the n-type In without forming the (1 × N) optical waveguide grating router 17 is used.
An undoped InGaAsP cladding layer (bandgap wavelength: λ _g = 1.1 μm, thickness: 0.5 μm) 27 is formed on the P substrate 26 by metal organic chemical vapor deposition (MOVPE). Next, as shown in FIG. 4B, as an active layer,
Selective growth technology (Aoki et al., Microwave and Optical Technology Letter, 7, 132 pages (1
994)), the InGaAsP multiple quantum well layer 28 in which the thickness of the quantum well layer is spatially changed in a tapered shape.
To form Thus, the gain peak position of each amplification region 21 can be spatially changed in the plane. Next, as shown in FIG.
After the growth of the aAsP multiple quantum well layer 28, an undoped InGaAsP cladding layer (forbidden bandwidth: λ _g =
1.1 μm, thickness: 0.5 μm) 29, p-type InP cap layer (λ _g = 0.92 μm, acceptor concentration: N _A)
= 5 × 10 ¹⁸ cm ⁻³ , thickness: 0.1 μm) 30.

Next, as shown in FIG. 4D, each amplification region is spatially (electrically) separated by chemical etching, and mirrors 23 and 24 are formed by cleavage. Finally, FIG.
As shown in (E), AuGeNi / Au and Au / Au
By depositing Zn, the n-side electrode 31,
The p-side electrode 32 is formed. An electric signal having a frequency equal to the frequency interval Δf of the (1 × N) optical waveguide grating router 17 is applied to the electrodes of each amplification region by the voltage pulse power supply 22, and mode locking is actively applied between the frequencies to shorten the period. An optical pulse train 25 is generated. According to this mode-locked semiconductor laser, a pulse train having a pulse width of about 10 fs to 100 fs can be generated.

<Embodiment 3> Here, FIGS. 5 and 6
Passive synchronous type consisting of an optical waveguide grating router and a spatially (electrically) separated optical amplification medium equal in number to the wavelength (frequency) spatially demultiplexed by the optical waveguide grating router An example of a mode-locked laser will be described. FIG. 5 is a plan view of the mode-locked semiconductor laser, and FIG. 6 is a cross-sectional view for explaining a method of forming a saturable absorption region.

N-type InP on the (100) crystal orientation (forbidden bandwidth: λ _g = 0.92 μm, donor concentration: N _D = 2)
(× 10 ¹⁸ cm ⁻³ ) InGaAsP on a substrate (1
× N) An optical waveguide grating router 33 is formed. This (1 × N) optical waveguide grating router 3
3 spatially decomposes the N wavelength (frequency) components λ _{1 to} λ _N (f _{1 to} f _N ) of the optical pulse incident from the optical waveguide 34 into an optical waveguide 36, and decomposes them into respective optical waveguides 36. To the amplifying region 37 (A _{1 to} A _N ) connected to.
Therefore, each wavelength component has a different amplification region 37 (A ₁
~ A _N ). Here, the frequency interval Δf = f
_{k + 1−} f _k (k = 1 to N−1) is constant.

Each of the amplification regions 37 (A _{1 to} A _N ) is formed in the same manner as in the first embodiment described with reference to FIG. The method for forming the amplification region 37 (A _{1 to} A _N ) may be the method described in the second embodiment. Next, a method of forming the saturable absorption region 42 at the end of the optical waveguide 34 will be described with reference to FIG. First, as shown in FIG. 6A, an undoped InGaAs is formed on an n-type InP substrate 44.
A P cladding layer (bandgap wavelength: λ _g = 1.1 μm, thickness: 0.5 μm) 45 is formed. Next, as shown in FIG. 6B, an InGaAsP multiple quantum well layer 46 is formed. Further, as shown in FIG. 6C, an undoped InGaAsP cladding layer (forbidden bandwidth wavelength: λ _g
= 1.1 μm, thickness: 0.5 μm) 47, and p-type In
P cap layer (λ _g = 0.92 μm, acceptor concentration: N _A = 5 × 10 ¹⁸ cm ⁻³ , thickness: 0.1 μm) 48
To form Next, as shown in FIG.
The P cap layer 48 is removed by chemical etching except for the portion where the electrode is to be formed.

After the amplification region 37 and the saturable absorption region 42 are formed, the mirrors 39 and 40 are formed by cleavage.
Finally, as shown in FIG. 6E, AuGeNi / Au
And Au / AuZn by vapor deposition, respectively.
The side electrode 49 and the p-side electrode 50 are formed in the saturable absorption region 42. When the electrodes 49 and 50 are formed in the saturable absorption region 42, as described with reference to FIG.
7, an n-side electrode 15 and a p-side electrode 16 are simultaneously formed.

In this mode-locked semiconductor laser, the amplification region 37 is forward-biased by a constant-voltage power supply 38, and the saturable absorption region 42 is reverse-biased by a constant-voltage power supply 43 to operate as a passive mode-locked laser. 41 is generated. Further, an electric signal having a frequency equal to the frequency interval Δf of the (1 × N) optical waveguide grating router 33 is applied to the saturable absorption region 42, and a short optical pulse train 41 can be generated by operating as a hybrid mode-locked laser. . According to this mode-locked semiconductor laser, 1
A pulse train having a pulse width of about 0 fs to 100 fs can be generated.

In the above description, the electric signal is applied to the amplification region or the saturable absorption region. However, the same operation can be performed by applying an optical signal instead of the electric signal. The mode-locked semiconductor laser of the present invention is used for a long-distance large-capacity optical communication system using a time division multiplex communication system, a wavelength division multiplex communication system, or a hybrid communication system of the time division multiplex communication and the wavelength division multiplex communication system as a pillar, or It is suitable as an ultrashort optical pulse generator that is indispensable for ultrahigh-speed optical measurement.

[0030]

According to the present invention, since a gain band exceeding the limit of the gain bandwidth of a semiconductor can be realized, the pulse width and repetition frequency of a conventional semiconductor mode-locked laser can be drastically improved. .

[Brief description of the drawings]

FIG. 1 is a plan view showing an example of a mode-locked semiconductor laser according to the present invention.

FIG. 2 is a cross-sectional view illustrating a method for forming an amplification region.

FIG. 3 is a plan view showing another example of the mode-locked semiconductor laser according to the present invention.

FIG. 4 is a sectional view showing a method for forming an amplification region.

FIG. 5 is a plan view showing another example of the mode-locked semiconductor laser according to the present invention.

FIG. 6 is a cross-sectional view illustrating a method for forming a saturable absorption region.

FIG. 7 is a configuration sectional view of a typical conventional active mode-locked semiconductor laser.

8A and 8B are diagrams illustrating a basic operation of the mode-locked laser, wherein FIG. 8A is a diagram of a pulse train, and FIG. 8B is a diagram illustrating a relationship between a gain spectrum and a longitudinal mode.

FIGS. 9A and 9B are diagrams for explaining the amplifying operation of the conventional mode-locked semiconductor laser, wherein FIG. 9A is a basic configuration diagram of the laser, and FIG.
FIG. 4 is a diagram showing a relationship between a gain spectrum and a longitudinal mode.

FIGS. 10A and 10B are diagrams illustrating an amplifying operation of a mode-locked semiconductor laser according to the present invention, wherein FIG. 10A is a diagram illustrating a basic configuration of the laser, and FIG.

[Explanation of symbols]

1 ... (1 × N) optical waveguide grating router, 2 ...
Optical waveguide, 3 ... coupling optical waveguide, 4 ... optical waveguide, 5 ... optical waveguide, 6 ... voltage pulse power supply, 7 ... mirror, 8 ... mirror,
9 ... light pulse train, 10 ... n-type InP substrate, 11 ... InG
aAsP cladding layer, 12... InGaAsP active layer, 1
3 ... InGaAsP cladding layer, 14 ... p-type InP cap layer, 15 ... n-side electrode, 16 ... p-side electrode, 17 ... (1
× N) Optical waveguide grating router, 18: Optical waveguide, 19: Coupling optical waveguide, 20: Optical waveguide, 21: Amplification region, 22: Voltage pulse power supply, 23: Mirror, 24: Mirror, 25: Optical pulse train, 26: n-type InP substrate, 27
... InGaAsP cladding layer, 28 ... Tapered InGa
AsP multiple quantum well layer, 29 ... InGaAsP cladding layer, 30 ... p-type InP cap layer, 31 ... n-side electrode, 3
2 ... p-side electrode, 33 ... (1 x N) optical waveguide grating router, 34 ... optical waveguide, 35 ... coupling optical waveguide, 3
6 optical waveguide, 37 amplification region, 38 constant voltage power supply, 3
9 ... mirror, 40 ... mirror, 41 ... light pulse train, 42 ...
Saturable absorption region, 43: constant voltage power supply, 44: n-type InP
Substrate, 45 ... InGaAsP cladding layer, 46 ... InG
aAsP multiple quantum well layer, 47 ... InGaAsP cladding layer, 48 ... p-type InP cap layer, 49 ... n side electrode,
50: p-side electrode, 51: crystal substrate, 52: clad layer,
53 ... active layer, 54 ... p-side electrode, 55 ... n-side electrode, 56
... voltage pulse power supply, 57 ... light pulse, 58 ... gain spectrum, 59 ... vertical mode, 60 ... mirror, 61 ... vertical mode, 62 ... optical amplification medium, 63 ... voltage pulse power supply, 64 ...
Gain spectrum, 65: mirror, 66: duplexer, 67:
Optical amplifying medium, 68: voltage pulse power supply, 69: longitudinal mode,
70: gain spectrum, 71: mask

Claims (9)

[Claims] An optical element for splitting an incident optical pulse into a plurality of frequency components and spatially separating and outputting the frequency components, and individually amplifying each frequency component spatially separated and output from the optical element. A mode-locked laser, comprising: a plurality of optical amplifying media having different peak positions in a gain band. 2. The plurality of frequency components have equal frequency intervals, and the plurality of optical amplifying media individually amplify the frequency components incident on each optical amplifying medium. Mode-locked laser. 3. An optical element for splitting an optical pulse incident on one input optical waveguide into a plurality of frequency components having equally spaced frequency intervals and outputting each frequency component from a plurality of different output optical waveguides, A mode-locked laser comprising: a plurality of electrically separated optical amplification media coupled to each output optical waveguide of an optical element. 4. The optical amplification medium according to claim 1, wherein an electric signal or an optical signal having a frequency equal to an integral multiple or a fraction of an integral frequency interval of said plurality of frequency components is applied to said optical amplification medium. The mode-locked laser as described. 5. The mode-locked laser according to claim 1, wherein the mode-locked laser has a saturable absorption region. 6. A mode-locked laser, wherein an electric signal or an optical signal having a frequency equal to an integral multiple or a fraction of a frequency interval of the plurality of frequency components is applied to the saturable absorption region. 7. A time-division multiplexed optical communication system using the mode-locked laser according to claim 1 as a short pulse light source. 8. A wavelength division multiplexing optical communication system using the mode-locked laser according to claim 1 as a short pulse light source. 9. A hybrid optical communication system in which a wavelength division multiplexing system and a time division multiplexing system are mixed, wherein the mode-locked laser according to claim 1 is used as a short pulse light source.