JP2001352123A - Semiconductor short-pulse light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To expand a longitudinal mode interval and to achieve the high repetition of [THz] class in the length of a resonator for obtaining a sufficient gain for stable oscillation in a mode synchronous type laser in resonator structure using SSG regarding a semiconductor short-pulse light source. SOLUTION: This semiconductor short-pulse light source is provided with a mode synchronous type laser that allows SSG2A, a saturable absorption waveguide 4B, a gain waveguide 4A, and SSG2B to be arranged in series in a described order where SSG2A and 2B existing at both ends have structure where the directions and periods of a phase change match.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in a semiconductor short pulse light source suitable as a light source for optical communication.

[0002]

2. Description of the Related Art In recent years, the number of Internet users has been rapidly increasing, and as a result, optical communication systems have been developed at a rapid pace.

[0003] At present, 10 [G bits / second] is used for trunk communication.
However, the demand for new services that can receive large-scale data such as moving images from a remote location at any time is increasing, so technology to further increase communication volume dramatically Is needed.

[0004] Optical time division multiplexing (optical ti)
me-division multiplexing:
OTDM) is different from time-division multiplexing based on conventional electronic circuits in that multiplexing approaching 1 [THz] is performed using a plurality of ultrashort pulse light sources and an all-optical multiplexing system. To achieve this, an ultrashort pulse light source using a semiconductor is essential.

[0005] In addition, since the ultrashort pulse light source itself has a high repetition characteristic, a path for increasing capacity utilizing conventional time division multiplex communication is opened.

As a short pulse light source using a semiconductor,
A mode-locked laser that integrates a gain waveguide and a saturable absorption waveguide in series and uses a Fabry-Perot type resonance mode due to reflection of the waveguide end face is known. SSG (super structure)
A mode-locked laser using a re-grating reflector has also been proposed.
No. 5733).

[0007] The repetition frequency of a mode-locked laser is determined by adjacent mode intervals in the longitudinal mode determined by the resonator structure.

In a Fabry-Perot resonator having a resonator length L, the mode interval is Δω = πc / nL (c: speed of light,
n: refractive index), and the repetition frequency at this time is f =
Δω / 2π = c / 2nL.

Therefore, in order to realize a high repetition in the Fabry-Perot type, the length of the resonator must be reduced. In this case, however, it is difficult to obtain a sufficient gain for oscillation. Assuming a standard semiconductor waveguide of about 0.5, the resonator length for obtaining a repetition of 1 [THz] is 43 [μm].

Further, the time width of each pulse is limited by the intensity ratio of each longitudinal mode oscillation light that contributes to mode synchronization. That is, the Fabry-Perot type is limited by the wavelength dependence of the gain in the gain waveguide, and it is difficult to obtain a sufficiently short pulse.

The above-mentioned Japanese Patent Application Laid-Open No. 335733/1991 proposes a structure using SSG as a reflecting mirror. In the invention, the time required for the light of each mode to circulate in the resonator is adjusted. Is not taken into account,
It can be easily imagined that the realization of the mode period would be difficult.

By the way, at first glance, the laser constituting the short pulse light source according to the present invention may be recognized as being similar to the laser according to the above-mentioned proposed invention. A more detailed description may be helpful in clarifying the novelty of the present invention.

In the invention disclosed in the above publication, the SSG structure is used as a reflecting mirror on one end face of the laser, and the other end face is made of a normal, that is, a highly reflective film whose reflectivity does not depend on the wavelength. It works as a synchronous laser.

However, in the SSG structure, since the effective reflection point differs depending on the wavelength, if this is used alone, the cavity length changes depending on the wavelength, and each mode Is difficult to synchronize.

In order to synchronize the modes, the group velocity dispersion of the waveguide must be appropriately designed and the resonator circulating time of each mode must be uniform. This group velocity dispersion is a function of the current injection amount. In the laser disclosed in the above publication, the tolerance for the operating condition for realizing the mode locking is very small.

[0016]

In the present invention, the mode locking of the resonator structure using the SSG mirror is used. However, the resonator length at which a sufficient gain for stably oscillating is obtained is obtained. The present invention seeks to provide means for increasing the vertical mode interval and realizing [THz] class high repetition.

[0017]

In general, when the SSG structure is used, in the case of the Fabry-Perot type, the frequency is an integral multiple (m times) of the longitudinal mode interval determined by the resonator length. It is possible to create reflectors with high reflectivity at intervals.

FIG. 1 is a cutaway side view showing a principal part of a mode-locked laser for explaining the principle of a semiconductor short pulse light source according to the present invention. In FIG. 1, reference numeral 1 denotes a substrate, 2A and 2B denote resonator ends. SSG 3 is a cladding layer, 4A is a gain waveguide, 4B is a saturable absorbing waveguide, 5 is a cladding layer, and 6 is an electrode.

In the present invention, the SSGs 2A and 2B are arranged as reflectors at both ends of the resonator, and are operated as a resonator structure having a mode interval m times the longitudinal mode interval determined by the length of the resonator. Partially saturable absorbing waveguide 4B
Is provided to cause passive mode locking, and an m-times high repetition pulse train is obtained corresponding to the expansion of the mode interval.

FIGS. 2A and 2B are diagrams showing the relationship between the mode interval and the repetition frequency. FIG. 2A shows the case of the present invention, and FIG. 2B shows the case of a Fabry-Perot resonator. As described above, Δω is a mode interval.

In the present invention, the SSGs 2A and 2B disposed at both ends of the resonator extend the tolerance to the operating conditions for realizing mode locking by aligning the period and direction of the phase change.

That is, light of a certain wavelength is reflected near the end face by the action of the SSG formed on one side, and is reflected near the gain waveguide by the action of the SSG formed on the other side. You.

Contrary to the above, light of another wavelength is reflected near the saturable absorbing waveguide by the action of the SSG formed on one side, and is reflected by the SSG formed on the other side. The light can be reflected in the vicinity of the end face by the action, and therefore, it has a function of making the resonator length uniform regardless of the wavelength, so that mode locking can be easily realized.

FIGS. 3A and 3B are main part explanatory views showing a laser for explaining reflection of light having different wavelengths. FIG. 3A is a diagram showing a conventional example and FIG. 3B is a diagram showing a conventional example shown for comparison. In this case, the same symbols as those used in FIG. 1 represent the same parts or have the same meaning.

As can be seen, in the laser according to the invention,
It will be clearly seen that the cavity lengths are aligned for light of different wavelengths, and that this is not possible with conventional lasers.

Further, according to the present invention, the time width of each pulse can be shortened as compared with the case of using the conventional technique, by utilizing the characteristics of SSG.

FIG. 4 is a diagram for explaining the flattening of the oscillating light intensity of each mode due to the design of the SSG.
(A) shows the gain spectrum, (B) shows the SSG reflection characteristic, and (C) shows the oscillation spectrum.

In general, the SSG can control the wavelength dependence of the reflectance by appropriately designing it. For example, in the laser of the present invention shown in FIG.
It is easy for the SG 2A and the SSG 2B on the other side to have a reflection characteristic that cancels out the wavelength dependence of the gain in the gain waveguide 4A, and by doing so, as shown in FIG. If the light intensities of the individual longitudinal modes are made uniform, the pulse time width can be shortened as compared with the conventional laser by ideal mode locking.

FIG. 5 is a diagram showing the intensity distribution of each longitudinal mode light and the time waveform of the synchronizing pulse. The solid line in the diagram showing the time waveform of the illustrated pulse shows the waveform according to the present invention. Also, broken lines indicate waveforms according to the conventional technique.

As described above, the repetition frequency f in the mode-locked laser is determined by the interval Δω between adjacent longitudinal modes when viewed on the frequency axis (or the wavelength axis).
Δω / 2π, and therefore, in order to realize high repetition, it is essential to increase the vertical mode interval.

Normally, in a mode-locked laser using a Fabry-Perot resonator, the longitudinal mode interval is determined by the length of the resonator, and when mode locking occurs, it depends on the interference of a plurality of light beams of different modes. In such a situation, only one light pulse is moving in the resonator, and the time that light circulates around the resonator is a repetition period.

On the other hand, when only light having a wavelength of an integer multiple of the longitudinal mode interval determined by the length of the resonator is synchronized, a situation in which a train of a plurality of optical pulses moves in the resonator is realized. This makes it possible to obtain a high repetition pulse train without shortening the resonator.

In the present invention, an SSG structure is used as a mechanism for selecting the wavelength.
Is a pattern in which a diffraction grating with a gradually changed pitch is periodically formed. In addition to the periodicity of the basic diffraction grating structure, a larger period (super period) is provided, so that a plurality of wavelengths can be obtained. (Refer to “H. Ishii, et. Al, IEEE J.Q., if necessary.)
E-32. 3. pp 433 (1996)).

It is easy to set the high reflection wavelength at an integer multiple interval of the Fabry-Perot mode, whereby a highly repetitive mode-locked laser can be realized.

Here, it should be noted that:
When the SSG structure is used, the effective reflection point is distributed depending on the wavelength. Therefore, when the SSG is provided only on one side of both end faces in the laser, the resonance between the longitudinal modes is caused. The captain will be different.

In general, in mode synchronization, it is essential to match the orbiting times of the respective mode lights.
When the SSG is provided only on one side, fine adjustment of the group velocity dispersion of the entire waveguide is required, and this group velocity dispersion is determined by the structure of the waveguide and the amount of injected current. Mode synchronization is realized only at a certain fixed amount of current.

Further, in addition to the strict tolerance of the synchronization condition, a structure is obtained in which only a light output of a predetermined intensity can be extracted.

However, in the laser constituting the short pulse light source of the present invention, since the SSGs having the same phase are provided on both end faces, the effective resonator lengths of the respective modes are uniform, and therefore, the circulating laser is not required. Since the time is ready, all of the above problems are eliminated.

Further, the time width of the pulse generated by the mode locking changes depending on the ratio of the light intensity of each mode.
It is known that the shortest is obtained when the light intensities of all the modes match.

The highly reflective film, which is frequently used in the Fabry-Perot type, does not depend on the wavelength spread of the mode contributing to the synchronization. Therefore, the light intensity ratio of each mode is determined by the gain waveguide. Is determined by the wavelength dependence of the gain at
Since the gain of the semiconductor has a Gaussian distribution with respect to the wavelength, it cannot satisfy the ideal mode-locking condition.

In the SSG, it is possible to freely design the wavelength dependence of the reflectance (if necessary, “Kumino Fumiyoshi, IEICE 4th Photonic Switching Study Group PS96-9 (1996)”) ), It is possible to provide a reflectivity that cancels the gain, so that the light intensity of each longitudinal mode can be made uniform to realize ideal mode-locking conditions, and to achieve the ultra-high Fourier transform limit. Short pulses can be generated.

According to the basic principle of the present invention described above, a mode-locked laser stably generating a [THz] class high repetition short pulse train can be easily realized. Can be an ultrashort pulse up to the Fourier transform limit.

[0043]

FIG. 6 to FIG. 10 are main part explanatory views for explaining steps in the case of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention. Among them, (A) or the one without display shows the cut surface of the main part, and (B) shows the plane of the main part.

FIG. 6 6- (1) By applying a resist process in the lithography technique, a resist layer is formed on the n-InP substrate 11, and then an SSG is to be formed using an electron beam exposure apparatus. The portion is exposed to a striped pattern, and then developed. The stripe pattern in this case is a pattern in which the pitch is periodically changed.

6- (2) The InP substrate 11 is etched using the above-mentioned resist layer as a mask by applying a dry etching method using an etching gas
2A and SSG12B are formed.

The SSGs 12A and 12B are designed to have a high reflectance with a period of Δω = 2πc / nL, where n _eff is the effective refractive index of the resonator structure formed thereafter and L is the length of the resonator. It is.

FIG. 7 7- (1) MOCVD (metalorganic chemical)
The clad layer 13, the SCH layer 14, the multiple quantum well layer 15, the SCH layer 16, the clad layer 17, and the contact layer 18 are grown by applying an al deposition method.

The following is an example of main data relating to each of the grown semiconductor layers. (1) cladding layer 13 material: n-InGaAsP bandgap wavelength lambda _g: 1.23 [μm] impurity concentration: 5 × 10 ¹⁷ [cm ^-3] Thickness: 70 [nm] (2) SCH layer 14 material: n-InGaAsP band gap wavelength λ _g : 1.15 [μm] Impurity concentration: 1 × 10 ¹⁴ [cm ^-3 ] Thickness: 30 [nm] (3) Multiple quantum well layer 15 barrier layer Material: i-InGaAsP band gap wavelength lambda _g: 1.3 [μm] thickness: 10 [nm] well layer material: i-InGaAsP bandgap wavelength lambda _g: 1.78 [μm] thickness: 5 nm, period: 10 (4) SCH layer 16 material: p-InGaAsP bandgap wavelength lambda _g: 1.15 [μm] impurity concentration: 1 × 10 ¹⁴ [cm ^-3] thickness: 100 [nm] (5) cladding layer 17 material: p-InP Impurity concentration: 5 × 10 ¹⁷ [cm ⁻³ ] Thickness: 50 [nm] (6) Contact layer 18 Material: p-InGaAsP Band gap wavelength λ _g : 1.3 [μm] Impurity concentration: 5 × 10 ¹⁷ [cm ⁻³ ] Thickness: 50 [nm]

8- (1) By applying a resist process in the lithography technique, the center of the surface of the contact layer 18 in the longitudinal direction is approximately 1.5 [μm] wide. A stripe resist film extending in a direction perpendicular to the irregularities such as SSGs 12A and 12B is formed. Note that FIG.
This is a vertical side surface at a position off the central stripe in FIG.

8- (2) Etching from the surface of the contact layer 18 to the surface of the cladding layer 13 using the stripe resist film as a mask by applying a dry etching method using an ethane-based etching gas. To form a stripe ridge.

8- (3) By applying the MOCVD method, a high-resistance layer 19 made of InP doped with iron (Fe) is formed in a portion other than the ridge and flattened.

9- (1) Resist process in lithography technology and
By applying a dry etching method using an ethane-based etching gas, the contact layer 18 is cut in a direction perpendicular to the stripe, and a portion corresponding to the SSG 12A, a portion corresponding to the saturable absorbing waveguide, and a gain waveguide. Resistance separation (air separation) is performed for each of the corresponding portion and the SSG12B corresponding portion.

10- (1) The p-side electrode 21 made of a Ti / Pt / Au film is formed on the p-side by applying the vacuum evaporation method and the lithography technique.
Then, an Au.Ge/Au film is formed on the n-side to form an n-side electrode 22.

In the laser fabricated as described above, when a current is injected into the gain waveguide, self-excited oscillation is accelerated by saturation absorption in the saturable absorbing waveguide, and a short pulse train is formed by passive synchronization. Can be generated.

The present invention can be embodied in many forms, including the above-described embodiment.

(Supplementary Note 1) Super-periodic diffraction grating (for example, SSG
2A: FIG. 1, the same applies hereinafter), a saturable absorbing waveguide (for example, saturable absorbing region 4B), a gain waveguide (for example, gain waveguide 4A), and a super-periodic diffraction grating (for example, SSG2B) are arranged in series in the above order. The semiconductor short pulse light source is provided, wherein the super-periodic diffraction gratings provided at both ends are provided with a mode-locked laser having a structure in which the directions and the periods of the phase changes match. (1)

(Supplementary note 2) The semiconductor short pulse light source according to (Supplementary note 1), wherein the gain waveguide is a quantum well structure waveguide. (2)

(Supplementary note 3) The semiconductor short pulse light source according to (Supplementary note 1), wherein the gain waveguide is a quantum box structure waveguide.

(Supplementary Note 4) In (Supplementary note 1), the wavelength dependence of the reflectance in the super-periodic diffraction grating and the wavelength dependence of the gain in the gain waveguide are contradictory. Semiconductor short pulse light source. (3)

[0060]

According to the semiconductor short pulse light source of the present invention, a super-periodic diffraction grating, a saturable absorbing waveguide, a gain waveguide, and a super-periodic diffraction grating are arranged in series in the above-described order. The super-period diffraction grating includes a mode-locked laser having a structure in which the direction and the period of the phase change match.

According to the above configuration, a mode-locked laser stably generating a [THz] class high repetition short pulse train can be easily realized, and the time width of each pulse can be Fourier transformed. Ultrashort pulses up to the limit can be obtained.

[Brief description of the drawings]

FIG. 1 is a cutaway side view showing a main part of a mode-locked laser for explaining the principle of a semiconductor short pulse light source according to the present invention.

FIG. 2 is a diagram illustrating a relationship between a mode interval and a repetition frequency.

FIG. 3 is an explanatory view of a main part of a laser for describing reflection of light having different wavelengths.

FIG. 4 is a diagram for explaining flattening of the oscillation light intensity of each mode due to the design of the SSG.

FIG. 5 is a diagram illustrating an intensity distribution of each longitudinal mode light and a time waveform of a synchronization pulse.

FIG. 6 is an explanatory view of a main part for describing a process in the case of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention.

FIG. 7 is an explanatory view of a principal part for describing a step of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention.

FIG. 8 is an explanatory view of a main part for describing a step of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention.

FIG. 9 is an explanatory view of a main part for describing a step of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention.

FIG. 10 is an explanatory view of a main part for describing a process in the case of manufacturing a mode-locked laser constituting a semiconductor short pulse light source according to an embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 11 n-InP substrate 12A and 12B SSG 13 clad layer 14 SCH layer 15 multiple quantum well layer 16 SCH layer 17 clad layer 18 contact layer 19 high resistance layer 21 p-side electrode 22 n-side electrode

Claims (3)

[Claims] 1. A super-period diffraction grating, a saturable absorbing waveguide, a gain waveguide, and a super-period diffraction grating are arranged in series in the above order, and the super-period diffraction gratings at both ends are directed in the direction of phase change. And a mode-locked laser having a structure in which the periods coincide with each other. 2. The semiconductor short pulse light source according to claim 1, wherein the semiconductor gain waveguide is a quantum well structure waveguide. 3. The semiconductor short pulse according to claim 1, wherein the wavelength dependence of the reflectance in the super-periodic diffraction grating and the wavelength dependence of the gain in the gain waveguide are contradictory. light source.