JP4444742B2 - Tunable semiconductor mode-locked laser - Google Patents

Description

  The present invention relates to a wavelength tunable semiconductor mode-locked laser, and is useful, for example, when it is desired to control the repetition frequency and center wavelength of an optical pulse independently and accurately within a predetermined range.

  With the increase in the number of channels in a wavelength division multiplexing (WDM) communication system, there is an increasing expectation for a tunable mode-locked laser as a multi-wavelength light source capable of simultaneously oscillating multiple wavelengths.

  In conventional tunable mode-locked lasers using chirped gratings, the longitudinal mode spacing is wavelength dependent by using a chirped grating fabricated in a fiber or semiconductor waveguide to make the resonator length wavelength dependent. However, the center frequency is selected so that the modulation frequency directly applied to the optical modulator section or gain section provided in the resonator matches the longitudinal mode interval of the resonator. By changing it, it becomes possible to change the center wavelength (for example, see Non-Patent Documents 1, 2, and 3).

J. Yu, D. Huhse, M. Schell, M. Schulze, D. Bimberg, JAR Williams, L. Zhang, and I. Bennion, "Fourier-transform-limited 2.5ps light pulses with electrically tunable wavelength (15nm) by hybridly modelocking a semiconductor laser in a chirped Bragggrating fiber external cavity, "Electron. Lett., vol. 31, pp. 2008-2009, 1995. S. Li and K. T. Chan, "Electrical wavelength-tunable actively mode-locked fiber ring laser with a linearly chirped fiber Bragg grating," IEEE Photon. Technol. Lett., Vol. 10, pp. 799-801, 1998. K. Sato, H. Ishii, I. Kotaka, Y. Kondo, and M. Yamamoto, "Frequency Range Extension of Actively Mode-Locked Lasers Integrated with Electroabsorption Modulators Using Chirped Gratings," IEEE J. Sel. Top. Quantum Electron, vol. 3, no. 2, pp. 250-252, 1997.

  In such a case, since the modulation frequency must be changed to change the center wavelength, it has been difficult to control the center wavelength and the modulation frequency independently, but using two chirped gratings, By providing means for changing the refractive index of each grating region, the modulation frequency and the center wavelength can be controlled independently, and the variable range of the center wavelength can be increased in principle. However, even in a wavelength tunable semiconductor mode-locked laser having such a configuration, there are ripples (variations) in the reflectance and effective length of the two chirped gratings, so that there is variation in the optical spectrum intensity, and the It was difficult to select the center wavelength.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a wavelength-tunable semiconductor mode-locked laser capable of suppressing the variation in optical spectrum intensity and selecting the center wavelength accurately.

  The wavelength-tunable semiconductor mode-locked laser according to the present invention that solves the above problems is characterized by the following points.

1) a semiconductor gain section having a gain with respect to a predetermined wavelength range; a semiconductor optical modulator section in which a light absorption coefficient or an optical gain coefficient with respect to the wavelength range changes by applying a voltage or current; and the predetermined wavelength range In a wavelength tunable semiconductor mode-locked laser in which a distributed Bragg reflector portion (hereinafter referred to as a DBR portion) having a grating having a reflection band is formed on the same substrate,
The DBR section is disposed at two positions so as to sandwich the semiconductor gain section and the semiconductor modulator section, and each DBR section is composed of a chirped grating whose period of the grating changes as a function of position, and A so-called apodization is performed in which the coupling coefficient of the grating is gradually increased along the traveling direction of the incident light, maximized in the vicinity of the center of the DBR portion, and then gradually decreased.

2) In the wavelength tunable semiconductor mode-locked laser described in 1),
The apodization applied to the DBR portion is realized by changing the depth of the unevenness in the grating.

3) In the wavelength tunable semiconductor mode-locked laser described in 2),
The DBR portion is formed as a so-called vertical grating type DBR by using a vertical grating that forms a grating perpendicular to the side surface of the waveguide, and the depth of the unevenness is changed.

4) In the wavelength tunable semiconductor mode-locked laser described in 1),
The apodization applied to the DBR portion is realized by inserting a portion without a grating and changing the number of irregularities per unit length.

5) In the wavelength tunable semiconductor mode-locked laser described in 1),
The apodization applied to the DBR portion is realized by changing the ratio of the grating period to the length of the convex portion of the grating.

  According to the present invention, by applying apodization to the chirped grating of the distributed Bragg reflector section, it is possible to realize a wavelength tunable semiconductor mode-locked laser capable of suppressing the variation in optical spectrum intensity and selecting the center wavelength accurately.

A wavelength tunable semiconductor mode-locked laser according to the present invention includes a semiconductor gain unit, a semiconductor optical modulation unit, and DBR units disposed at two locations so as to sandwich the semiconductor gain unit and the semiconductor modulation unit. This DBR portion is made of chirped grating, and further, so-called apodization is applied in which the coupling coefficient of the grating gradually changes along the traveling direction of incident light and becomes the largest near the center of the DBR portion. .
DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic structural diagram showing a basic configuration of a wavelength tunable semiconductor mode-locked laser according to an embodiment of the present invention.
As shown in FIG. 1, a wavelength tunable semiconductor mode-locked laser according to the present invention includes a semiconductor gain section (hereinafter abbreviated as a gain section) 1 having a gain with respect to a predetermined wavelength range, and the wavelength by applying a voltage. Semiconductor optical modulator section (hereinafter abbreviated as modulator section) 2 in which the light absorption coefficient with respect to the region increases, and distributed Bragg reflector sections (hereinafter referred to as DBR sections) 3 and 4 having gratings on the same semiconductor substrate The DBR portions 3 and 4 are formed at two locations so as to sandwich the gain portion 1 and the modulator portion 2. Non-reflective films 5 and 6 are disposed on both end faces. The modulator unit 2 may change the optical gain coefficient with respect to a predetermined wavelength region by applying a current.

  The DBR units 3 and 4 are chirped gratings in which the grating period changes as a function of position. Specifically, one DBR portion 3 is composed of a chirped grating configured such that the period of the grating decreases as it goes to the end face side, and the other DBR portion 4 includes the grating as it goes to the end face side. It consists of a chirped grating configured to increase the period. Such a chirped grating is formed, for example, in one DBR section 3 by inserting a plurality of constant negative phase shifts and reducing the phase shift insertion interval toward the end face side. The other DBR portion 4 can be formed by inserting a plurality of constant positive phase shifts and decreasing the phase shift insertion interval toward the end face side.

  Further, in the DBR units 3 and 4, apodization is performed in which the coupling coefficient of the grating is gradually increased along the traveling direction of the incident light, maximized in the vicinity of the center of the DBR units 3 and 4, and then gradually decreased. . Specifically, the size of the unevenness in the grating is gradually increased along the traveling direction of the incident light, is maximized near the centers of the DBR portions 3 and 4, and is gradually decreased thereafter, or the unit length. This is realized by gradually increasing the number of the concavo-convex portions along the traveling direction of the incident light, increasing it most near the center of the DBR portion, and then gradually decreasing it.

  FIG. 2 is a calculation example of the wavelength dependence of the reflectance and the effective length in the chirped grating with and without apodization. The effective length represents the effective length of the grating, and the wavelength dependency of the effective length corresponds to the wavelength dependency of the resonator length described later. As is clear from FIG. 2, it can be seen that the apodization significantly reduces the reflectivity and effective length ripple in the chirped grating.

  Fig. 3 (a) is a calculation example of the wavelength dependence of the mirror loss and the longitudinal mode interval when a resonator is configured using a chirped grating subjected to apodization, and Fig. 3 (b) shows the calculation of the apodization. This is a calculation example of the wavelength dependence of the mirror loss and the longitudinal mode interval when a resonator is configured using a non-chirped grating.

The mirror loss and the longitudinal mode interval are expressed as follows.
Mirror loss: α _DBR = (1 / L _act ) ln (1 / (R ₁ R ₂ ) ^1/2 ) (1)
Longitudinal mode _{spacing: Δf = c / 2n g L} (2)
Here, L _act is the length of the gain region, R ₁ R ₂ is the product of the reflectivities in the two DBR sections, c is the speed of light, _ng is the group refractive index, and L is the resonator length. Since R ₁ R ₂ and L have wavelength dependency, the mirror loss and the longitudinal mode interval also have wavelength dependency.

  In the wavelength range from 1535 nm to 1565 nm in FIGS. 3A and 3B, the mirror loss has a substantially constant value, so that the modulation frequency applied to the modulator section and the longitudinal mode interval are matched in the wavelength range. The correct wavelength is selected as the center wavelength. For example, in FIG. 3A, when the modulation frequency is 25.5 GHz, the center wavelength is about 1550 nm. On the other hand, in FIG. 3B, when the modulation frequency is 25.5 GHz, a plurality of selected wavelengths are irregularly present around 1550 nm due to the effect of ripple of the effective length in the chirped grating. The variation in the spectral intensity appears remarkably, and the center wavelength cannot be selected accurately.

  Based on the above, by applying apodization to the chirped grating, it becomes possible to realize a wavelength-tunable semiconductor mode-locked laser that can suppress variation in optical spectrum intensity and can select the center wavelength accurately.

  FIG. 4 is a schematic structural diagram showing an example of a wavelength tunable semiconductor mode-locked laser according to an embodiment of the present invention. This is a schematic view of the wavelength tunable semiconductor mode-locked laser according to the present embodiment as viewed from vertically above the substrate. In this embodiment, an example of a mode-locked laser with a 1.55 μm wavelength band and a modulation frequency of around 25 GHz is shown. However, the wavelength band can be changed by changing the composition ratio or composition of the semiconductor, and the length of the resonator can be changed. It is easy to change the modulation frequency band by changing.

  The wavelength tunable semiconductor mode-locked laser according to the present embodiment has the configuration shown in FIG. 1, specifically, two locations so as to sandwich the gain section 1, the modulator section 2, and the gain section 1 and the modulator section 2. The DBR units 3 and 4 are arranged in the above structure, and the DBR units 3 and 4 are made of chirped gratings subjected to apodization. Specifically, the DBR portions 3 and 4 are vertical grating type DBRs in which a grating is formed perpendicular to the side surface of the waveguide, and apodization is realized by changing the size of the irregularities of the grating. ing.

  The gain section 1 is composed of a multiple quantum well in which the well layer is InGaAs or InGaAsP and the barrier layer is InGaAsP, and the photoluminescence wavelength thereof is around 1.55 μm. The modulator section 2 and the DBR sections 3 and 4 are composed of multiple quantum wells in which the well layer is InGaAs or InGaAsP and the barrier layer is InGaAsP, and the photoluminescence wavelength thereof is around 1.49 μm.

  The gain section 1 is a ridge waveguide and has a length of about 1000 μm. The modulator unit 2 and the DBR units 3 and 4 are high-mesa waveguides. The modulator unit 2 has a length of 100 μm, the DBR unit 3 has a length of 330 μm, and the DBR unit 4 has a length of 500 μm. By applying a taper that gradually changes the waveguide width to the ridge waveguide and / or the high mesa waveguide, the coupling efficiency between the ridge waveguide and the high mesa waveguide can be increased. Although the gain section 1 is a ridge waveguide here, it may be a buried waveguide. Further, although the modulator unit 2 is a high mesa waveguide, it may be a ridge waveguide or a buried waveguide.

  As shown in FIG. 4, the DBR unit 3 is configured to insert a large number of negative phase shifts 7, 8, and 9, so that the grating period decreases as it goes to the end face side. The rate is 1.15 nm / (100 μm). Here, the chirp rate is expressed in the DBR portion in terms of the amount of change in the grating period per 100 μm. The DBR unit 4 is configured to insert a large number of positive phase shifts 10, 11, and 12 so that the period of the grating increases toward the end face side, and the chirp rate is 0.73 nm / (100 μm). ).

In addition, the DBR units 3 and 4 change the coupling coefficient κ according to the following expression by changing the size of the irregularities of the grating, thereby realizing apodization.
κ = κ ₀ sin (πz / L) (3)
Here, κ ₀ is the maximum value of the coupling coefficient, z is the distance from the light incident side, L is the length of the DBR, and κ ₀ is 80 cm ⁻¹ . The modulation function of the coupling coefficient may be a Gaussian function, a Sinc function, or the like that maximizes the center of the DBR portion other than the trigonometric function.

Next, wavelength tunable characteristics will be described.
FIG. 5 shows the wavelength dependence of the mirror loss and the longitudinal mode interval of the resonator. 5A shows a case where no current is injected into the DBR units 3 and 4. FIG. 5B shows a case where a current is injected into the DBR unit 3 and the refractive index of the DBR unit 3 is reduced by about 0.3%. In this case, FIG. 5C shows a case where current is injected into the DBR portion 4 and the refractive index of the DBR portion 4 is reduced by about 0.3%.

  When a modulation frequency of 25 GHz is applied to the modulator unit 2, when current injection into the DBR units 3 and 4 is not performed, the center wavelength is about 1546 nm from FIG. 5A, and current injection into the DBR unit 3 was performed. In this case, the center wavelength is about 1557 nm from FIG. 5B, and when current is injected into the DBR unit 4, the center wavelength is about 1530 nm from FIG. In this case, the influence of ripple is not seen, and the center wavelength can be accurately changed over the wavelength range of 25 to 30 nm.

  Unlike the case where no apodization is applied to the optical spectrum characteristics, there are no multiple wavelengths having a specific longitudinal mode interval, so the intensity decreases uniformly as the wavelength goes away from the center wavelength. A consistent light spectrum can be obtained.

  FIG. 6 is a schematic structural diagram showing another example of a wavelength tunable semiconductor mode-locked laser according to an embodiment of the present invention. This shows a schematic cross-sectional view of the wavelength-tunable semiconductor mode-locked laser of this example. In the present embodiment, similarly to the first embodiment, an example of a mode-locked laser having a 1.55 μm wavelength band and a modulation frequency of around 25 GHz is shown.

  The wavelength tunable semiconductor mode-locked laser of this embodiment is also arranged at two locations so as to sandwich the configuration shown in FIG. 1, that is, the gain unit 1, the modulator unit 2, and the gain unit 1 and the modulator unit 2. The DBR units 3 and 4 are made of chirped gratings subjected to apodization. Specifically, apodization applied to the DBR units 3 and 4 is realized by changing the number of irregularities per unit length.

  The gain section 1 is composed of a multiple quantum well in which the well layer is InGaAs or InGaAsP and the barrier layer is InGaAsP, and the photoluminescence wavelength thereof is around 1.55 μm. The modulator section 2 and the DBR sections 3 and 4 are composed of multiple quantum wells in which the well layer is InGaAs or InGaAsP and the barrier layer is InGaAsP, and the photoluminescence wavelength thereof is around 1.49 μm.

  The gain unit 1, the modulator unit 2, the DBR units 3 and 4 have a semi-insulating InP buried structure, and the length is 1000 μm for the gain unit 1, 100 μm for the modulator unit 2, 330 μm for the DBR unit 3, and 500 μm for the DBR unit 4 It is. Although the waveguide is a buried waveguide here, it may be a ridge waveguide.

As shown in FIG. 6, the DBR unit 3 is configured such that a large number of negative phase shifts 7, 8, 9 are inserted so that the period of the grating decreases toward the end face side. Is 1.15 nm / (100 μm). The DBR unit 4 is configured to insert a large number of positive phase shifts 10, 11, and 12 so that the period of the grating increases toward the end face side, and the chirp rate is 0.73 nm / (100 μm). ). In addition, the DBR portions 3 and 4 are inserted with portions 13 and 14 having no grating as shown in FIG. 6 to change the number of grating irregularities per unit length, so that the coupling coefficient κ is effectively reduced. In order to achieve apodization, the equation (3) was changed. The maximum value κ ₀ of the coupling coefficient was set to 80 cm ⁻¹ , as in Example 1.

  Under the above conditions, a wavelength tunable semiconductor mode-locked laser was constructed, and its wavelength dependence was examined in the same manner as in FIG. 5 of Example 1. As for wavelength tunable characteristics and optical spectrum characteristics, Example 1 also in this example. Similar characteristics were obtained.

  FIG. 7 is a structural diagram showing still another example of the wavelength tunable semiconductor mode-locked laser according to the embodiment of the present invention. This is for explaining the configuration of the DBR section in the wavelength tunable semiconductor mode-locked laser of this embodiment, specifically, the outline of apodization.

  The wavelength tunable semiconductor mode-locked laser of this example has the same structure as that of Example 2 except for the DBR part. Therefore, here, overlapping description is omitted, and the characteristic points of the present embodiment will be mainly described.

In FIG. 7, the direction from the light incident position to the end face direction is a positive direction of z, FIG. 7 (a) shows a schematic shape of the grating, and FIG. 7 (b) shows the refractive index of each region A1 to A6. The average value is shown, and FIG. 7C shows the average value of the coupling coefficient for each of the regions A1 to A6. As shown in FIG. 7, in the wavelength tunable semiconductor mode-locked laser of this embodiment, the DBR units 3 and 4 change the ratio of the grating period to the length of the convex portion of the grating in the DBR units 3 and 4. Apodization to be applied to is realized. Here, if the coupling coefficient is κ, the pitch of the grating is Λ, and the length of the convex portions A7 to A12 of the grating in FIG. 7A is W, the following relationship exists between κ, Λ, and W. is there.
κ∝sin (πW / Λ) (4)

  For example, by setting W / Λ to be 2/14, 12/14, 4/14, 10/14, 6/14, 8/14, etc. in the regions A1 to A6, respectively, as shown in FIG. As described above, the regions A1 and A2, the regions A3 and A4, and the regions A5 and A6 have the same coupling coefficient κ, and as is clear from the equation (4), κ It can be seen that apodization is realized because the change is made in accordance with Equation (3).

  Here, as shown in FIG. 7 (b), the refractive index repeats the magnitude, but this makes the average refractive index the same in the regions A1 and A2, the regions A3 and A4, and the regions A5 and A6. Thus, the effective period change of the grating accompanying the refractive index distribution is avoided.

1 is a schematic diagram showing a basic configuration of a wavelength tunable semiconductor mode-locked laser according to an embodiment of the present invention. It is a calculation example of the wavelength dependence of the reflectance and effective length in a chirped grating with and without apodization. (A) is a calculation example of the wavelength dependence of mirror loss and longitudinal mode spacing when a resonator is configured using a chirped grating subjected to apodization, and (b) is a chirped grating not subjected to apodization. This is a calculation example of the wavelength dependence of the mirror loss and the longitudinal mode interval when a resonator is configured. It is the schematic which shows an example (Example 1) of the wavelength tunable semiconductor mode-locked laser which concerns on embodiment of this invention. It is a graph which shows the wavelength dependence of the mirror loss and longitudinal mode space | interval of the wavelength tunable semiconductor mode-locked laser of Example 1. (A) shows no current injection into the DBR units 3 and 4, (b) shows current injection into the DBR unit 3, and when the refractive index of the DBR unit 3 is reduced by about 0.3%, (c ) Is a case where current is injected into the DBR portion 4 and the refractive index of the DBR portion 4 is reduced by about 0.3%. It is the schematic which shows another example (Example 2) of the wavelength tunable semiconductor mode locked laser which concerns on embodiment of this invention. It is a figure which shows the further another example (Example 3) of the wavelength-tunable semiconductor mode-locked laser which concerns on embodiment of this invention, and demonstrates the outline | summary of apodization. (A) is the schematic shape of the grating, (b) is the average value of the refractive index for each of the regions A1 to A6, and (c) is the average value of the coupling coefficient for each of the regions A1 to A6.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor gain part 2 Semiconductor optical modulator part 3, 4 DBR part (distributed Bragg reflector part)
5, 6 Non-reflective film 7, 8, 9 Negative phase shift 10, 11, 12 Positive phase shift 13, 14 Parts without grating

Claims (5)

A semiconductor gain section having a gain with respect to a predetermined wavelength range; a semiconductor optical modulator section in which a light absorption coefficient or an optical gain coefficient with respect to the wavelength range changes by applying a voltage or current; and a reflection to the predetermined wavelength range In a wavelength tunable semiconductor mode-locked laser in which a distributed Bragg reflector section having a grating with a band is formed on the same substrate,
The distributed Bragg reflector section is disposed at two locations so as to sandwich the semiconductor gain section and the semiconductor modulator section, and the grating period of any of the distributed Bragg reflector sections changes as a function of position. Apodization that consists of chirped grating and gradually increases the coupling coefficient of the grating along the traveling direction of the incident light, becomes the largest near the center of the distributed Bragg reflector, and then gradually decreases. A wavelength tunable semiconductor mode-locked laser. The wavelength tunable semiconductor mode-locked laser according to claim 1,
2. A wavelength tunable semiconductor mode-locked laser, wherein the apodization applied to the distributed Bragg reflector is realized by changing the depth of irregularities in the grating. In the wavelength tunable semiconductor mode-locked laser according to claim 2,
Wherein the distributed Bragg reflector portion, with a vertical grating for forming a grating perpendicular to the side surface of the waveguide, the wavelength tunable semiconductor mode-locked laser, wherein changing the depth of the irregularities. The wavelength tunable semiconductor mode-locked laser according to claim 1,
A wavelength tunable semiconductor mode-locked laser, wherein the apodization applied to the distributed Bragg reflector is realized by inserting a portion without a grating and changing the number of irregularities per unit length. The wavelength tunable semiconductor mode-locked laser according to claim 1,
2. A wavelength tunable semiconductor mode-locked laser, wherein apodization applied to the distributed Bragg reflector is realized by changing a ratio of a grating period to a length of a convex part of the grating.

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