JP4594816B2 - Tunable laser - Google Patents

Description

  The present invention relates to a semiconductor laser used as a light source for optical communication, and more particularly to a wavelength tunable laser capable of changing an oscillation wavelength over a wide range and at high speed.

Along with a dramatic increase in communication demand in recent years, a wavelength division multiplexing communication system (WDM communication system) that enables large-capacity transmission over a single optical fiber by multiplexing a plurality of signal lights having different wavelengths. Development is underway.
In such a wavelength division multiplexing communication system, in order to realize a flexible and advanced communication system, a wavelength tunable laser capable of selecting a desired wavelength at high speed in a wide wavelength range is strongly demanded.

For example, as a wavelength tunable laser capable of continuously changing the oscillation wavelength, a three-electrode DBR (Distributed Bragg Reflector) laser or a TTG-DFB (Tunable Twin Guide-Distributed Feedback) is used. ) Lasers have been proposed.
Here, as shown in FIG. 11, the three-electrode DBR laser 100 includes an active layer portion 101, a phase control portion 102, and a DBR portion 104 in which a diffraction grating 103 is formed along an optical waveguide. The active layer portion 101, the phase control portion 102, and the DBR portion 104 are arranged in series. The active layer portion 101, the phase control portion 102, and the DBR portion 104 are provided with electrodes 105, 106, and 107, respectively, so that current can be injected independently. Furthermore, a common electrode 108 connected to the ground potential is provided on the surface opposite to the surface on which these electrodes 105, 106, and 107 are provided. Then, the active layer 101 are injected current I _act through the electrode 105, current I _PS via the electrode 106 is injected into the phase control section 102, the DBR portion 104 through the electrode 107 current ( Wavelength control current) I _DBR is injected.

As shown in FIG. 12, the TTG-DFB laser 110 includes an active waveguide 111 that generates a gain by current injection and a wavelength control waveguide 112 that changes an oscillation wavelength by changing a refractive index by current injection. The active waveguide 111 is laminated on the wavelength control waveguide 112 through the intermediate layer 113. A diffraction grating 114 is formed along the active waveguide 111 and the wavelength control waveguide 112 over the entire length thereof. Further, an electrode 115 for injecting a current I _act into the active waveguide 111 is provided on the upper surface, and an electrode 116 for injecting a current I _tune into the wavelength control waveguide 112 is provided on the lower surface. Is provided. The intermediate layer 113 is connected to the ground potential. As a result, current injection can be performed independently for each of the active waveguide 111 and the wavelength control waveguide 112.

As a means for realizing a broadband wavelength tunable laser, for example, an array integrated wavelength tunable laser in which a plurality of wavelength tunable lasers having a wavelength tunable range of several nm to several tens nm is integrated on the same substrate has been proposed. Yes.
For example, Non-Patent Document 1 proposes a DBR laser integrated as a wavelength tunable laser. Patent Document 1 proposes an integrated TTG-DFB laser as a wavelength tunable laser.

In such an array-integrated wavelength tunable laser, in order to perform a wavelength tunable operation at a high speed in a wide wavelength range, the wavelength tunable range of each integrated wavelength tunable laser is widened and the wavelength tunable operation is accelerated. Is required.
For example, when the above-described three-electrode DBR laser 100 or TTG-DFB laser 110 is integrated as a wavelength tunable laser, the three-electrode DBR laser 100 or the TTG-DFB laser 110 is connected to the phase control unit 102 or the wavelength control waveguide 112. Since the oscillation wavelength can be changed by current injection, the wavelength can be changed at high speed (for example, 10 nanoseconds or less).

  On the other hand, it has been reported that the wavelength tunable range of individual wavelength tunable lasers integrated can be widened to about 10 nm in the case of DBR laser and about 7 nm in the case of TTG-DFB laser. In this case, by integrating 4 to 7 wavelength tunable lasers in one array integrated wavelength tunable laser, wavelength tunable operation is possible in the range of 1530 to 1560 nm (C band) which is important in the WDM communication system.

  By the way, in the DBR laser, when current (wavelength control current) is injected to change the oscillation wavelength, the Bragg wavelength and the longitudinal mode wavelength gradually shift, and mode jump (mode hopping) occurs. become. Therefore, in order to be able to continuously change the oscillation wavelength while preventing mode jumping, the phase control in which no diffraction grating is formed as in the above-described three-electrode DBR laser 100 is used. It is necessary to match the Bragg wavelength and the longitudinal mode wavelength by providing the unit 102 and injecting a current into the phase control unit 102.

However, in such a three-electrode DBR laser 100, in addition to the control of the reflection wavelength in the DBR unit 104, the phase control in the phase control unit 102 is necessary, so that the control becomes complicated.
Therefore, as a technique for eliminating the need for phase control, it has been proposed to devise the configuration of the electrode for injecting current into the distributed reflection region, the length of the active waveguide, and the length of the inactive waveguide for adjusting the phase. (For example, refer to Patent Document 2). In addition, the active region and the non-active region are alternately and periodically arranged along the light propagation direction, and the region where the diffraction grating is formed and the region where the diffraction grating is not formed are arranged with the same cycle. A structure has been proposed (see, for example, Patent Document 4).

As another wavelength tunable laser that is controlled in terms of current and can continuously change the oscillation wavelength without causing mode jump, for example, a multi-electrode DFB (Distributed Feed Back). Lasers have been proposed (see, for example, Non-Patent Document 2 and Patent Document 3).
JP 2004-235600 A JP 9-36480 A JP-A-4-147686 JP 7-273400 A ECOC2003 PROCEEDING vol4. Pp 887 (Th1.2.4) Electronics Letters 20th July 1989 vol25 No15, pp 990-992

  By the way, as described above, in the DBR laser, in addition to controlling the reflection wavelength in the DBR unit 104 in order to be able to continuously change the oscillation wavelength while preventing mode jumping, the phase is not limited. Phase control in the control unit 102 is also required. In this case, there are two wavelength control parameters, and the control is complicated. For this reason, it is difficult to perform wavelength control at high speed.

  In the technique described in Patent Document 2 described above, the same current is injected into the comb electrode for injecting current into the distributed reflection waveguide and the electrode in the inactive waveguide region for adjusting the phase. The resonance longitudinal mode wavelength and the Bragg wavelength can be changed at the same rate, but it is necessary to control the phase in order to make the resonance longitudinal mode wavelength and the Bragg wavelength coincide with each other. It is. In particular, in an array integrated wavelength tunable laser, control at the time of laser switching is complicated, and it is difficult to perform wavelength control at high speed.

  On the other hand, in the TTG-DFB laser, mode jump does not occur. However, as described above, the active waveguide 111 and the wavelength control are controlled so that currents can be injected independently into the active waveguide 111 and the wavelength control waveguide 112, respectively. Since it is necessary to provide the intermediate layer 113 between the waveguide 112 and the intermediate layer 113 to be connected to the ground potential, the fabrication of the element becomes complicated as compared with a normal laser. In particular, it is difficult to produce an array integrated wavelength tunable laser by integration.

Further, since the above-mentioned multi-electrode DFB laser has a wavelength tunable range of about 2 to 3 nm, it is necessary to integrate 10 or more lasers to cover all the important C bands in the WDM communication system. Considering the yield of the element, it is not realistic.
Further, with the technique described in Patent Document 4 described above, it is difficult to adjust the phase state of the region where the diffraction grating is not formed only by making the element. In addition, phase control is necessary and control is complicated.

  The present invention was devised in view of such problems, and an object of the present invention is to provide a wavelength tunable laser that can be easily manufactured and can obtain a relatively wide wavelength tunable range with simple control. .

For this reason, the wavelength tunable laser of the present invention includes an optical waveguide having alternately a gain waveguide section capable of generating gain and a wavelength control waveguide section capable of controlling the oscillation wavelength by current injection in the optical axis direction, A diffraction grating provided along the optical waveguide over the entire length of the waveguide, and the total length of the pair of gain waveguide portion and wavelength control waveguide portion constituting the optical waveguide injects current into the wavelength control waveguide portion. The continuous wavelength tunable width capable of continuously oscillating in one resonance longitudinal mode is larger than the oscillation wavelength tunable width in the case, and the wavelength control waveguide portion is formed of a semiconductor material. The ratio of the length of the pair of gain waveguide sections and wavelength control waveguide sections constituting the waveguide is 1: X (X> 0), and the total length of the pair of gain waveguide sections and wavelength control waveguide sections is , (140 + 30 × X) μm or less der Rukoto It is characterized by.

  The array integrated wavelength tunable laser of the present invention is an array integrated wavelength tunable laser comprising a plurality of wavelength tunable lasers having different wavelength tunable ranges on the same substrate, and each of the plurality of wavelength tunable lasers is the above-mentioned It is a tunable laser.

Therefore, according to the wavelength tunable laser of the present invention, there is an advantage that it can be easily manufactured and a relatively wide wavelength tunable range can be obtained with simple control. In addition, since control is simple, wavelength control can be performed at high speed.
In particular, fabrication of an array integrated wavelength tunable laser is facilitated. In addition, control at the time of laser switching is simplified, and wavelength control can be performed at high speed.

Hereinafter, a wavelength tunable laser according to an embodiment of the present invention will be described with reference to FIGS.
[First Embodiment]
First, a wavelength tunable laser according to a first embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the wavelength tunable laser according to the present embodiment (current control type wavelength tunable laser) controls the oscillation wavelength by a gain waveguide section 1A capable of generating a gain by current injection and a refractive index change by current injection. An optical waveguide (optical waveguide layer) 1 having a wavelength control waveguide portion 1B that can be used, and a diffraction grating (diffraction grating layer) 2 provided in the vicinity of the optical waveguide 1 are configured. In the wavelength tunable laser, a current (gain control current) I _act is injected into the gain waveguide section 1A to oscillate at a wavelength corresponding to the period of the diffraction grating 2. The oscillation wavelength can be controlled by injecting a current (wavelength control current) I _tune into the wavelength control waveguide section 1B.

  Here, as shown in FIG. 1, the optical waveguide 1 is configured so as to alternately have gain waveguide portions (active waveguide portions) 1A and wavelength control waveguide portions 1B in the optical axis direction. That is, the optical waveguide 1 includes a plurality of gain waveguide portions 1A and a plurality of wavelength control waveguide portions 1B, and these gain waveguide portions 1A and wavelength control waveguide portions 1B are periodically arranged on the same plane. Are alternately arranged in series. A specific configuration example of the gain waveguide portion 1A and the wavelength control waveguide portion 1B will be described later.

  As shown in FIG. 1, the diffraction grating 2 is provided below the optical waveguide 1 in parallel along the optical waveguide 1 over the entire length of the optical waveguide 1. That is, the diffraction grating 2 is continuously provided at a position corresponding to the gain waveguide section 1A and at a position corresponding to the wavelength control waveguide section 1B. As shown in FIG. 1, the diffraction grating 2 formed at a position corresponding to the gain waveguide section 1A is called a gain diffraction grating 2A, and is formed at a position corresponding to the wavelength control waveguide section 1B. The diffraction grating 2 is referred to as a wavelength control diffraction grating 2B.

  Thus, unlike the TTG-DFB laser, in this variable wavelength laser, as shown in FIG. 1, the gain waveguide portion 1A and the wavelength control waveguide portion 1B are arranged on the same plane. Therefore, the device can be easily manufactured. For example, as in the second embodiment described later, even when an array integrated wavelength tunable laser is manufactured by integration, it can be easily integrated.

In addition, this wavelength tunable laser has the same configuration as a general DFB laser, and is a type of DFB laser. Therefore, unlike the DBR laser, it is not necessary to perform phase control during wavelength tunable control, and the wavelength control current Simple wavelength control by only I _tune is possible. In the wavelength tunable laser, since the diffraction grating 2 is provided over the entire length of the optical waveguide 1, it is not necessary to control the initial phase.

In this wavelength tunable laser, as shown in FIG. 1, electrodes are independently provided for each region so that current can be injected independently into the gain waveguide portion 1A and the wavelength control waveguide portion 1B of the optical waveguide 1. 3A and 3B are provided.
That is, as shown in FIG. 1, a gain electrode (P-side electrode) 3A is formed on the upper surface of the gain waveguide portion 1A of the optical waveguide 1 via a contact layer 8A, and a common electrode (N-side) is formed below. Electrode) 3C is formed, and a current I _act can be injected into the active layer (gain layer, waveguide core layer) 6 of the gain waveguide section 1A. A wavelength control electrode (P-side electrode) 3B is formed on the upper surface of the wavelength control waveguide portion 1B of the optical waveguide 1 via a contact layer 8B, and a common electrode (N-side electrode) 3C is formed below. The current I _tune can be injected into the wavelength control layer (waveguide core layer, phase control layer) 9 of the wavelength control waveguide section 1B.

Here, the gain electrode 3A and the wavelength control electrode 3B are both configured as comb-shaped electrodes as shown in FIG.
Note that a region including the gain waveguide portion 1A, the gain diffraction grating 2A, the gain electrode 3A, and the common electrode 3C is referred to as a gain region 11A, and includes a wavelength control waveguide portion 1B, a wavelength control diffraction grating 2B, a wavelength control electrode 3B, A region composed of the common electrode 3C is referred to as a wavelength control region 11B.

Further, as shown in FIG. 1, a SiO ₂ film (passivation film) 10 is formed in a region where the contact layers 8A and 8B, the wavelength control electrode (P side electrode) 3B and the gain electrode (P side electrode) 3A are not formed. Is formed. That is, after the contact layers 8A and 8B are formed, the SiO ₂ film 10 is formed on the entire surface, only the SiO ₂ film 10 on the contact layers 8A and 8B is removed, and the P-side electrodes 3A and 8B are formed on the contact layers 8A and 8B. By forming 3B, the SiO ₂ film 10 is formed in a region where the contact layers 8A and 8B and the P-side electrodes 3A and 3B are not formed.

  In particular, as shown in FIGS. 1 and 2, in order to electrically isolate the gain region 11A and the wavelength control region 11B, a separation region (separation part) 11C is provided between the gain electrode 3A and the wavelength control electrode 3B. Provided. That is, the wavelength control electrode (P-side electrode) 3B, the gain electrode (P-side electrode) 3A, and the contact layers 8A and 8B are not formed in the upper region near the junction interface between the gain region 11A and the wavelength control region 11B. Thus, the separation part 11C is formed.

  By the way, in this embodiment, among the plurality of gain waveguide sections 1A and the plurality of wavelength control waveguide sections 1B, the total length of the pair of adjacent gain waveguide sections 1A and wavelength control waveguide sections 1B is the wavelength control. The continuous wavelength variable width capable of continuously oscillating in one resonance longitudinal mode is larger than the oscillation wavelength variable width when current is injected into the waveguide section 1B. In other words, when the gain waveguide sections 1A and the wavelength control waveguide sections 1B are alternately arranged periodically, one gain waveguide section 1A and one wavelength control waveguide section 1B have one cycle. The length of one period is larger than the oscillation wavelength variable width when current is injected into the wavelength control waveguide section 1B, so that the continuous wavelength variable width capable of continuously oscillating in one resonance longitudinal mode becomes larger. It is configured as follows.

  In particular, when the wavelength control waveguide portion 1B is made of a semiconductor material and the ratio of the lengths of the pair of adjacent gain waveguide portions 1A and wavelength control waveguide portions 1B is 1: X (X> 0), The total length of a pair of adjacent gain waveguide portions and wavelength control waveguide portions is set to be (140 + 30 × X) μm or less. In other words, when the gain waveguide sections 1A and the wavelength control waveguide sections 1B are alternately arranged periodically, one gain waveguide section 1A and one wavelength control waveguide section 1B have one cycle. The length of one cycle is set to be (140 + 30 × X) μm or less.

As a result, it is possible to continuously change the wavelength by making maximum use of the refractive index change that occurs in the wavelength control waveguide section 1B, and it is possible to maximize the continuous wavelength variable width. Details will be described below.
In this wavelength tunable laser, the gain region 11A has a constant Bragg wavelength, the refractive index of the core layer of the wavelength control waveguide section 1B in the wavelength control region 11B is changed, and the wavelength is changed by changing the Bragg wavelength in the wavelength control region 11B. Perform the action.

For example, when the length of the gain waveguide portion 1A and the length of the wavelength control waveguide portion 1B are 1: 1 (when X = 1), the oscillation wavelength of the wavelength tunable laser is the Bragg wavelength of the gain region 11A. This is the average value of the Bragg wavelength in the wavelength control region 11B.
For this reason, the Bragg wavelength (oscillation wavelength) λ _Bragg of the tunable laser when the refractive index of the core layer of the wavelength control waveguide section 1B is changed is set to λ _{a as} the Bragg wavelength of the gain region 11A. the Bragg wavelength of 11B as a lambda _t, the Bragg wavelength of the wavelength control region 11B of the front (current injection before) to change the refractive index and lambda _t0, the amount of change in equivalent refractive index of the wavelength controlling waveguide portion 1B and [Delta] n _t The equivalent refractive index of the wavelength control waveguide section 1B before changing the refractive index (before current injection) can be expressed by the following equation (1), where n _t is the equivalent refractive index.

λ _Bragg = (λ _a + λ _t ) / 2 = {λ _a + λ _t0 (1 + Δn _t / n _t )} / 2 (1)
Therefore, the change amount Δλ _Bragg of the Bragg wavelength (oscillation wavelength) of the present tunable laser can be expressed by the following equation (2).
Δλ _Bragg = λ _t0 · (Δn _t / n _t ) / 2 (2)
On the other hand, the ratio of the change in the resonance longitudinal mode wavelength (the position of the resonance longitudinal mode) is the ratio of the length of the total wavelength control waveguide section 1B to the total resonator length (the total length of the plurality of wavelength control waveguide sections 1B). Therefore, it becomes smaller than the rate of change in the equivalent refractive index of the wavelength control waveguide section 1B by 1/2 (here, 1/2).

Therefore, the amount of change Δλ _l of the resonance longitudinal mode wavelength can be expressed by the following equation (3), where λ ₀ is the resonance longitudinal mode wavelength (oscillation wavelength) before changing the refractive index (before current injection).
Δλ _l = λ ₀ · (Δn _t / n _t ) / 2 (3)
Therefore, from the above equations (2) and (3), if λ _t0 and λ ₀ are set to be substantially the same, the change amount Δλ _Bragg of the Bragg wavelength matches the change amount Δλ _l of the resonance longitudinal mode wavelength. I understand. Therefore, if λ _t0 and λ ₀ are set to be substantially the same, only the refractive index of the wavelength control waveguide section 1B is changed (that is, only the wavelength variable control is performed without performing the phase control). ), The oscillation wavelength can be continuously changed while preventing the mode jump. Note that λ _t0 and λ ₀ generally match substantially, but if it is desired to match completely, a λ / 4 phase shift unit 2C (see FIG. 5) may be provided.

By the way, in this wavelength tunable laser, when the difference between the Bragg wavelength in the wavelength control region 11B and the Bragg wavelength in the gain region 11A becomes too large during wavelength tunable control, mode jump occurs, and continuous wavelength tunable operation can be performed. Disappear.
That is, first, when a current is injected into the gain waveguide section 1A and no current is injected into the wavelength control waveguide section 1B, the peak (center wavelength; Bragg wavelength) of the reflection spectrum by the diffraction grating 2 in the wavelength control area 11B. ) Coincides with the peak (center wavelength; Bragg wavelength) of the reflection spectrum by the diffraction grating 2 in the gain region 11A, as indicated by the symbol A in FIG. In this case, the total reflection spectrum obtained by adding the reflection spectrum by the diffraction grating 2 in the wavelength control region 11B and the reflection spectrum by the diffraction grating 2 in the gain region 11A is as shown by reference numeral A ′ in FIG. The center wavelength (peak; Bragg wavelength) is the oscillation wavelength of the tunable laser.

  From this state, when current is injected into the wavelength control waveguide section 1B, the Bragg wavelength in the wavelength control region 11B shifts to the short wavelength side as shown in FIG. 3A, when the difference between the Bragg wavelength of the wavelength control region 11B and the Bragg wavelength of the gain region 11A increases, the total reflection spectrum is represented by the reference symbol in FIG. As indicated by B ', the width of the reflection spectrum is widened and has two peaks.

As described above, if the reflection spectrum of the diffraction grating 2 in each region is separated, oscillation at the center wavelength cannot be maintained, mode jump occurs, and continuous wavelength variable operation cannot be performed.
Therefore, as a result of intensive studies by the present inventor, as shown by a solid line A in FIG. 4, the width of a wavelength region capable of oscillation at the center wavelength of the total reflection spectrum (continuous wavelength variable width; without mode jumping) The width of the wavelength region that can be continuously oscillated in one resonance longitudinal mode) is the length of one cycle when one gain waveguide portion 1A and one wavelength control waveguide portion 1B are one cycle. It turned out to be almost inversely proportional to the above. FIG. 4 shows characteristics when the ratio of the length of the gain waveguide section 1A to the length of the wavelength control waveguide section 1B is 1: 1 and the oscillation wavelength band is 1.55 μm band.

For example, when the amount of change in oscillation wavelength (oscillation wavelength variable width) when current is injected into the wavelength control waveguide section 1B is below the solid line A in FIG. Although the oscillation wavelength can be continuously changed, mode jump occurs when it is above the solid line A in FIG.
In general, the rate of change in equivalent refractive index that can actually occur when current is injected into a semiconductor waveguide (maximum refractive index change rate) varies depending on the structure of the waveguide, but the optical waveguide is formed of a semiconductor material. For example, it is about 0.5%. This corresponds to the amount of change in the equivalent refractive index (refractive index change) in which the oscillation wavelength is changed by about 7 nm in the DBR laser and the TTG-DFB laser.

During current injection, 0.5% rate of change of the equivalent refractive index of the wavelength controlling waveguide section 1B (i.e., [Delta] n _t / n _t of 0.5%) When obtained, the wavelength controlling region 11B current injection before When the Bragg wavelength is in the 1.55 μm band (that is, λ _t0 is 1550 nm), the amount of change in the oscillation wavelength (Δλ _Bragg ) of the wavelength tunable laser, that is, the oscillation wavelength variable width is It becomes about 3.8 nm.

In order to perform the continuous wavelength tunable operation in the entire wavelength tunable range of the oscillation wavelength, the fundamental continuous wavelength tunable width of the wavelength tunable laser is set to 3.8 nm (in FIG. 4, It may be wider than that indicated by a solid line B).
In order to realize this, the length of one cycle may be set to 170 μm or less so that the continuous wavelength variable width is above the solid line B in FIG. If the length of one period is set to 170 μm or less in this way, the continuous wavelength variable width can be set to 3.8 nm or more, and the change in the refractive index caused by injecting current into the wavelength control waveguide section 1B can be reduced. It is possible to perform continuous wavelength variable operation by making the maximum use.

  In other words, if the length of one cycle is set to be longer than 170 μm, the oscillation wavelength can be changed to about 3.8 nm by changing the refractive index by injecting current into the wavelength control waveguide section 1B. Even if possible, mode jumping occurs with a wavelength variation smaller than 3.8 nm. On the other hand, if the length of one cycle is set to 170 μm or less, continuous wavelength variable operation is performed by making maximum use of the change in refractive index caused by injecting current into the wavelength control waveguide section 1B. Will be able to.

  In short, the total length of the pair of gain waveguide section 1A and wavelength control waveguide section 1B is continuous in one resonance longitudinal mode than the oscillation wavelength variable width when current is injected into the wavelength control waveguide section 1B. If the continuous wavelength tunable width that can be oscillated is increased, continuous wavelength tunable operation is performed by making maximum use of the change in refractive index caused by injecting current into the wavelength control waveguide section 1B. Will be able to.

Hereinafter, a specific configuration example will be described.
First, the gain region (active region) 11A includes, for example, an n-type InP buffer layer, an n-type InGaAsP diffraction grating layer, an n-type InP buffer layer, and a band gap wavelength of 1.55 μm on an n-type InP substrate (semiconductor substrate). 1.55 μm-band strained MQW layer (Multiple Quantum Well) + SCH (Separate Confinement Heterostructure) layer (InGaAsP layer), p-type InP cladding layer, p-type InGaAsP contact layer, p-type InGaAs It has a layer structure in which contact layers are sequentially laminated.

  That is, as shown in FIG. 1, the gain region 11A includes an n-InP layer (n-type InP substrate, n-type InP buffer layer) 4, an n-type InGaAsP diffraction grating layer 2, an n-type InP layer 5, an MQW active layer ( 1.55 μm-band strained MQW layer + SCH layer, waveguide core layer) 6, p-InP layer (p-type InP clad layer) 7, and contact layer (p-type InGaAsP contact layer, p-type InGaAs contact layer) 8A are laminated in this order. It has a layered structure.

The gain waveguide portion (active waveguide portion) 1 </ b> A includes an n-type InP layer 5, an MQW active layer 6, and a p-InP layer 7.
On the other hand, the wavelength control region 11B includes, for example, an n-type InP buffer layer, an n-type InGaAsP diffraction grating layer, an n-type InP buffer layer, a 1.38 μm composition InGaAsP layer (waveguide core layer), a p-type on an n-type InP substrate. It has a layer structure in which an InP clad layer, a p-type InGaAsP contact layer, and a p-type InGaAs contact layer are sequentially stacked.

  That is, as shown in FIG. 1, the wavelength control region 11B includes an n-InP layer (n-type InP substrate, n-type InP buffer layer) 4, an n-type InGaAsP diffraction grating layer 2, an n-type InP layer 5, and a wavelength control layer. (Phase control layer; 1.38 μm composition InGaAsP core layer) 9, p-InP layer (p-type InP clad layer) 7, and contact layer (p-type InGaAsP contact layer, p-type InGaAs contact layer) 8 are laminated in this order. It has become.

The wavelength control waveguide section 1B is composed of an n-type InP layer 5, a wavelength control layer 9, and a p-InP layer 7.
The diffraction grating layer 2 is formed by laminating a layer made of a material for forming the diffraction grating layer 2 on the n-InP layer 4, and then periodically removing the layer using a method such as dry etching. It is formed by growing an n-InP layer 5 thereon.

As the current confinement structure to the MQW active layer 6 and the wavelength control layer 9, for example, a pn-BH structure (Buried Heterostructure) may be used.
Here, in the wavelength control waveguide 1B, the material composition and thickness of the core layer are adjusted so that the equivalent refractive index becomes equal to the equivalent refractive index of the gain waveguide portion 1A.
In particular, the lengths of the gain waveguide portion 1A and the wavelength control waveguide portion 1B are both 30 μm, and the length of one period is 60 μm. Thereby, the fundamental continuous wavelength variable width of the wavelength variable laser becomes about 12 nm (see FIG. 4). The element length is, for example, 570 μm. Here, the gain waveguide portion 1A is arranged on the element end face side so that the optical output does not decrease. However, the wavelength control waveguide portion 1B may be disposed on the element end face side.

The period of the diffraction grating 2 is about 240 nm, for example, and the oscillation wavelength is in the 1.55 μm band.
With this configuration, the fundamental continuous wavelength tunable width of the tunable laser can be reduced to about 12 nm. Therefore, the wavelength tunable width (3) that can be caused by the refractive index change when current is injected into the semiconductor waveguide. It is possible to perform a continuous wavelength variable operation using the maximum of about .8 nm).

As shown in FIG. 5, the diffraction grating 2 is preferably configured to include a λ / 4 phase shift unit 2C at the longitudinal center position. That is, it is preferable to shift (phase shift) by half the period of the diffraction grating 2 (1/4 of the Bragg wavelength) at the center position in the longitudinal direction of the diffraction grating 2 as shown in FIG.
If the λ / 4 phase shift unit 2C is not provided, as shown in FIG. 6A, the center of the reflection spectrum by the diffraction grating 2 [the gain spectrum of the wavelength tunable laser; indicated by the solid line A in FIG. 6A] There is a possibility that it does not oscillate at the wavelength (peak; Bragg wavelength), but oscillates in the two neighboring modes [resonance longitudinal mode wavelength; indicated by the solid line B in FIG. 6 (A)]. In this case, it becomes unstable because it is not known whether oscillation occurs in the long wave side mode or the short wave side mode of the two modes.

  On the other hand, when the λ / 4 phase shift unit 2C is provided, as in a general DFB laser, as shown in FIG. 6B, the reflection spectrum by the diffraction grating 2 [the gain spectrum of the wavelength tunable laser; The center wavelength (peak: Bragg wavelength) of [shown as a solid line A] in (B) matches one of the resonance longitudinal mode wavelengths [shown by the solid line B in FIG. 6 (B)] so that oscillation occurs at the center wavelength. Therefore, stable single mode oscillation is possible.

However, even if the λ / 4 phase shift unit 2C is not provided, it normally oscillates in one of the two modes.
Therefore, the wavelength tunable laser according to the present embodiment has an advantage that it can be easily manufactured and a relatively wide wavelength tunable range can be obtained with simple control. In addition, since control is simple, wavelength control can be performed at high speed. In particular, since it is a current-controlled tunable laser, it has excellent high-speed response.

In the above-described embodiment, the case where the ratio of the length of the gain waveguide portion 1A to the length of the wavelength control waveguide portion 1B is 1: 1 (in the case of X = 1) is described as an example. The ratio of the length of the waveguide section 1A and the length of the wavelength control waveguide section 1B is not limited to this, and substantially the same operation and effect can be obtained even if the ratio is other ratio.
That is, when the length of the wavelength control waveguide section 1B is increased, when the same refractive index change as that in the ratio of 1: 1 is generated, the amount of change in the oscillation wavelength increases, but at the same time, the continuous wavelength is variable. The width also becomes wider. On the other hand, when the length of the wavelength control waveguide section 1B is shortened, when the same refractive index change as in the ratio 1: 1 is generated, the amount of change in the oscillation wavelength is small, but at the same time, the continuous wavelength is variable. The width is also narrowed. For this reason, regardless of the ratio of the length of the gain waveguide section 1A and the length of the wavelength control waveguide section 1B, the same operation and effect as when the ratio is 1: 1 can be obtained.

More specifically, when the ratio of the length of the pair of gain waveguide sections 1A and the wavelength control waveguide section 1B is 1: X (X> 0), the pair of gain waveguide sections 1A and the wavelength control The total length of the waveguide portion 1B may be (140 + 30 × X) μm or less.
First, when the ratio of the lengths of the pair of gain waveguide portions 1A and the wavelength control waveguide portion 1B is 1: X (X> 0), as the value of X increases (that is, the wavelength control waveguide portion) As the length of 1B increases, the amount of change in oscillation wavelength increases when current is injected into the wavelength control waveguide section 1B to cause the same amount of refractive index change. That is, as the value of X increases, the wavelength variable width (oscillation wavelength variable width) of the oscillation wavelength when current is injected into the wavelength control waveguide section 1B increases.

Here, FIG. 7 shows the gain waveguide portion 1A when the ratio of change in the equivalent refractive index of the wavelength control waveguide portion 1B is 0.5% when current is injected into the wavelength control waveguide portion 1B. FIG. 6 is a diagram showing the relationship between the ratio of the length of the wavelength control waveguide section 1B and 1: X (X> 0) and the wavelength variable width.
Here, the length of the gain waveguide section 1A is set to act, the length of the wavelength control waveguide section 1B is set to tune, and the ratio of these is shown as the value of tune / act. However, if the length act of the gain waveguide section 1A is 1, the length tune of the wavelength control waveguide section 1B is X, so the value of tune / act is X.

  As can be seen from FIG. 7, for example, when X = 1, that is, when the ratio of the length of the gain waveguide section 1A to the length of the wavelength control waveguide section 1B is 1: 1 (tune / act = 1). The wavelength variable width is about 3.8 nm, for example, when X = 2, that is, the ratio of the length of the gain waveguide section 1A to the length of the wavelength control waveguide section 1B is 1: 2. In the case (tune / act = 2), the wavelength variable width is about 5.2 nm, and it can be seen that the wavelength variable width increases as the value of X increases.

On the other hand, as the value of X becomes larger (that is, as the length of the wavelength control waveguide section 1B becomes longer), the continuous wavelength variable width that can change the wavelength while preventing mode jumping is also provided. There is a tendency to grow.
Here, FIG. 8 shows a ratio 1: X (X> 0) (here, the value of the tune / act) of the length of the gain waveguide section 1A and the wavelength control waveguide section 1B from 0.50 to 2.50. It is a figure which shows the relationship between the sum total length (length of 1 period) of a pair of gain waveguide part 1A and the wavelength control waveguide part 1B at the time of changing to 0.25 increments, and a continuous wavelength variable width.

As can be seen from FIG. 8, when the length of one period is the same (that is, when the lengths of the pair of gain waveguide portions 1A and the wavelength control waveguide portion 1B are the same), the larger the value of X (ie, It can be seen that the continuous wavelength variable width tends to increase as the value of tune / act increases; the length of the wavelength control waveguide section 1B increases.
By the way, as in the case where the ratio of the length of the gain waveguide section 1A and the wavelength control waveguide section 1B is 1: 1 (see FIG. 4), the continuous wavelength variable width causes a current to flow through the wavelength control waveguide section 1B. A pair of gain waveguide sections 1A and a wavelength control waveguide which are larger than the oscillation wavelength variable width when the ratio of change in the equivalent refractive index of the wavelength control waveguide section 1B is 0.5% when. When the total length of the waveguide portion 1B (the length of one cycle) is obtained, as shown in FIG. 9, the total length of the pair of gain waveguide portions 1A and the wavelength control waveguide portion 1B (the length of one cycle) It can be seen that Y can be approximated by the equation Y = (140 + 30 × X) (μm).

  That is, as shown in FIG. 9, the total length (length of one cycle) Y (μm) of a pair of gain waveguide section 1A and wavelength control waveguide section 1B having the same continuous wavelength variable width and oscillation wavelength variable width. ) (Allowable maximum value) is obtained, and this is associated with the length ratio 1: X (X> 0) (here, the value of tune / act) of the gain waveguide portion 1A and the wavelength control waveguide portion 1B. When plotted, it can be seen that the approximation can be made by the equation Y = (140 + 30 × X).

Therefore, if the total length (length of one cycle) of the pair of gain waveguide section 1A and wavelength control waveguide section 1B is set to (140 + 30 × X) μm or less, current is injected into the wavelength control waveguide section 1B. Thus, the continuous wavelength variable operation can be performed by making maximum use of the change in the refractive index caused by this.
In the above-described embodiment, the lengths of the plurality of gain waveguide portions 1A and the lengths of the plurality of wavelength control waveguide portions 1B are all the same, and the lengths of the respective periods are all set. However, the length of the longest period (the total length of the gain waveguide section 1A and the wavelength control waveguide section 1B with the longest period) satisfies the above-mentioned condition. It should be satisfied.

In the above-described embodiment, the description has been made on the assumption that the wavelength tunable laser has an oscillation wavelength band of 1.55 μm. However, the present invention is not limited to this. For example, the present invention can also be applied to a wavelength tunable laser in another oscillation wavelength band such as a 1.3 μm band.
That is, if the oscillation wavelength band is different, the characteristic of the continuous wavelength variable width with respect to the length of one period (see FIG. 4) changes, but the maximum change ratio of the equivalent refractive index of the wavelength control waveguide section 1B is 0.5%. Since the change amount of the oscillation wavelength in this case also changes, if the length of one cycle is set to (140 + 30 × X) μm or less as in the above-described embodiment, current is injected into the wavelength control waveguide portion 1B. Thus, the continuous wavelength variable operation can be performed by making maximum use of the change in the refractive index caused by the above.

Eventually, regardless of the oscillation wavelength band, the total length of the pair of gain waveguide section 1A and wavelength control waveguide section 1B is less than the oscillation wavelength variable width when current is injected into the wavelength control waveguide section 1B. If the continuous wavelength variable width capable of continuously oscillating in the resonance longitudinal mode is increased, the change in the refractive index caused by injecting current into the wavelength control waveguide section 1B can be utilized to the maximum. Thus, continuous wavelength variable operation can be performed.
[Second Embodiment]
Next, a wavelength tunable laser according to a second embodiment of the present invention will be described with reference to FIG.

As shown in FIG. 10, the wavelength tunable laser according to the present embodiment is an array integrated wavelength tunable laser, in which a plurality of the wavelength tunable lasers of the first embodiment described above are integrated in one element.
That is, as shown in FIG. 10, the wavelength tunable laser includes a plurality (eight here) of wavelength tunable lasers 20A to 20H having different wavelength tunable ranges and a plurality (eight here). The curved waveguides 21 </ b> A to 21 </ b> H, an optical combiner 22, and an optical amplifier (semiconductor optical amplifier) 23 are provided.

  Here, each of the wavelength variable lasers 20A to 20H is configured to have a predetermined continuous wavelength variable range of, for example, 6 nm or more. Thereby, a wavelength tunable laser having a wavelength tunable range of 40 nm can be realized with one element. As a result, a wavelength tunable laser capable of covering the entire range of 1530 to 1560 nm (C band) important in the WDM communication system can be realized.

Each of the wavelength tunable lasers 20A to 20H is configured such that the oscillation wavelength is different by, for example, 5 nm in a state where no current injection or voltage is applied to the wavelength control waveguide portion.
These wavelength tunable lasers 20 </ b> A to 20 </ b> H are connected to an optical amplifier 23 via a plurality of bent waveguides 21 </ b> A to 21 </ b> H and an optical combiner 22, respectively.
The plurality of bent waveguides 21A to 21H and the optical combiner 22 are configured to have the same layer structure as that of the wavelength control region of the wavelength tunable lasers 20A to 20H (see the above-described first embodiment). The optical amplifier 23 is configured to have a layer structure (see the first embodiment described above) similar to the gain region of the wavelength tunable lasers 20A to 20H.

Therefore, since the wavelength tunable laser integrated in the array integrated wavelength tunable laser according to the present embodiment is the wavelength tunable laser according to the first embodiment described above, it can be easily manufactured and has a relatively wide wavelength with simple control. There is an advantage that a variable range can be obtained, and there is also an advantage that wavelength control can be performed at high speed because the control is simple. Therefore, there is an advantage that it is easy to manufacture the array integrated wavelength tunable laser of this embodiment. In addition, there is an advantage that the control at the time of laser switching becomes simple and wavelength control can be performed at high speed. [Others]
In each of the above embodiments, the InGaAsP material is described as being used. However, the present invention is not limited to this. For example, other semiconductor materials such as InGaAlAs and GaInNAs can be used. The same effect can be obtained.

In addition, this invention is not limited to the structure described in each embodiment mentioned above, A various deformation | transformation is possible in the range which does not deviate from the meaning of this invention.
(Appendix 1)
An optical waveguide having alternately a gain waveguide section capable of generating gain and a wavelength control waveguide section capable of controlling the oscillation wavelength by current injection in the optical axis direction;
A diffraction grating provided along the optical waveguide over the entire length of the optical waveguide;
The total length of the pair of gain waveguide portion and wavelength control waveguide portion constituting the optical waveguide is one resonance longitudinal mode than the oscillation wavelength variable width when current is injected into the wavelength control waveguide portion. A wavelength tunable laser comprising a continuous wavelength tunable width that can be continuously oscillated.

(Appendix 2)
An optical waveguide having alternately a gain waveguide section capable of generating gain and a wavelength control waveguide section capable of controlling the oscillation wavelength by current injection in the optical axis direction;
A diffraction grating provided along the optical waveguide over the entire length of the optical waveguide;
The wavelength control waveguide portion is formed of a semiconductor material;
The ratio of the length of the pair of gain waveguide sections and wavelength control waveguide sections constituting the optical waveguide is 1: X (X> 0), and the total length of the pair of gain waveguide sections and wavelength control waveguide sections Is a wavelength tunable laser, characterized in that it is (140 + 30 × X) μm or less.

(Appendix 3)
A gain electrode for injecting current into the gain waveguide portion;
A wavelength control electrode for injecting current into the wavelength control waveguide portion;
The wavelength tunable laser according to appendix 1 or 2, wherein the gain electrode and the wavelength control electrode are provided independently of each other.

(Appendix 4)
The wavelength tunable laser according to appendix 3, wherein the gain electrode and the wavelength control electrode are both comb-type electrodes.
(Appendix 5)
5. The wavelength tunable laser according to any one of appendices 1 to 4, wherein the diffraction grating includes a λ / 4 phase shift unit at a longitudinal center position.

(Appendix 6)
An array integrated wavelength tunable laser comprising a plurality of wavelength tunable lasers having different wavelength tunable ranges on the same substrate,
6. The array-integrated wavelength tunable laser, wherein each of the plurality of wavelength tunable lasers is the wavelength tunable laser according to any one of appendices 1 to 5.

1 is a schematic cross-sectional view showing a configuration of a wavelength tunable laser according to a first embodiment of the present invention. It is a typical top view for demonstrating the structure of the electrode of the wavelength variable laser concerning 1st Embodiment of this invention. FIGS. 3A and 3B are views for explaining the state of wavelength tuning of the wavelength tunable laser according to the first embodiment of the present invention. It is a figure which shows the relationship between the length of 1 period and the continuous wavelength variable width of the wavelength variable laser concerning 1st Embodiment of this invention. It is a schematic diagram which shows the structure in the case of providing a (lambda) / 4 phase shift part in the wavelength tunable laser concerning 1st Embodiment of this invention. 6A and 6B are diagrams for explaining the reason why it is preferable to provide the λ / 4 phase shift unit in the wavelength tunable laser according to the first embodiment of the present invention. It is a figure which shows the relationship between the ratio of the length of a pair of wavelength control waveguide part and gain waveguide part in the wavelength variable laser concerning 1st Embodiment of this invention, and a wavelength variable width. It is a figure which shows the relationship between the length of 1 period in the wavelength tunable laser concerning 1st Embodiment of this invention, continuous wavelength variable width, and the ratio of the length of a pair of wavelength control waveguide part and a gain waveguide part. . It is a figure which shows the relationship between the ratio of the length of a pair of wavelength control waveguide part and gain waveguide part in the wavelength tunable laser concerning 1st Embodiment of this invention, and the maximum value of the length of 1 period. It is a schematic diagram which shows the structure of the array integrated wavelength variable laser concerning 2nd Embodiment of this invention. It is typical sectional drawing which shows the structure of the conventional 3 electrode DBR laser. It is typical sectional drawing which shows the structure of the conventional TTG-DFB laser.

Explanation of symbols

1 Optical Waveguide 1A Gain Waveguide (Active Waveguide)
1B Wavelength control waveguide 2 Diffraction grating (Diffraction grating layer)
2A Gain diffraction grating 2B Wavelength control diffraction grating 2C λ / 4 phase shift unit 3A Gain electrode (P-side electrode)
3B Wavelength control electrode (P side electrode)
3C common electrode (N side electrode)
4 n-InP layer 5 n-InP layer 6 MQW active layer (gain layer, active layer, waveguide core layer)
7 p-InP layer 8A, 8B Contact layer 9 Wavelength control layer (phase control layer)
10 SiO ₂ film 11A Gain region (active region)
11B Wavelength control region 11C Separation region 20A-20H Tunable laser 21A-21H Curved waveguide 22 Optical combiner 23 Optical amplifier

Claims (6)

An optical waveguide having alternately a gain waveguide section capable of generating gain and a wavelength control waveguide section capable of controlling the oscillation wavelength by current injection in the optical axis direction;
A diffraction grating provided along the optical waveguide over the entire length of the optical waveguide;
The total length of the pair of gain waveguide portion and wavelength control waveguide portion constituting the optical waveguide is one resonance longitudinal mode than the oscillation wavelength variable width when current is injected into the wavelength control waveguide portion. The continuous wavelength variable width that can be continuously oscillated is increased ,
The wavelength control waveguide portion is formed of a semiconductor material;
The ratio of the length of the pair of gain waveguide sections and wavelength control waveguide sections constituting the optical waveguide is 1: X (X> 0), and the total length of the pair of gain waveguide sections and wavelength control waveguide sections Saga, characterized in der Rukoto (140 + 30 × X) μm or less, a tunable laser. 2. The wavelength tunable laser according to claim 1, wherein a length of the wavelength control waveguide section is longer than a length of the gain waveguide section . A gain electrode for injecting current into the gain waveguide portion;
A wavelength control electrode for injecting current into the wavelength control waveguide portion;
3. The wavelength tunable laser according to claim 1, wherein the gain electrode and the wavelength control electrode are provided independently of each other.   4. The wavelength tunable laser according to claim 3, wherein each of the gain electrode and the wavelength control electrode is a comb electrode.   The wavelength tunable laser according to claim 1, wherein the diffraction grating includes a λ / 4 phase shift unit at a longitudinal center position. An array integrated wavelength tunable laser comprising a plurality of wavelength tunable lasers having different wavelength tunable ranges on the same substrate,
The array-integrated wavelength tunable laser, wherein each of the plurality of wavelength tunable lasers is the wavelength tunable laser according to any one of claims 1 to 5.

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