JP3219983B2 - Variable wavelength element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength tunable element which can be used as a semiconductor laser capable of varying an oscillation wavelength or a wavelength filter capable of varying a selected wavelength.

[0002]

2. Description of the Related Art As a conventional example of a wavelength tunable element of this kind, for example, a document I (Photonics Thechono
Logic Letters), Vol. 5, No. 6, pp. 613-615 (1993 June)). This laser
It has a DBR type structure in which a first super-period grating, an active region, a phase adjustment region, and a second super-period grating having a different period from the first super-period grating are cascaded. (For example, Reference I
FIG. 3). Further, each of the first super-period grating, the active region, the phase adjustment region, and the second super-period grating has a structure controlled by individual electrodes. The first and second two gratings are:
Since a comb-like reflection peak is shown for each wavelength, the vernier scale effect can be used. Therefore, in this wavelength tunable laser, a wide wavelength tunable characteristic is realized.

[0003]

However, in this structure, the active region exists between the two gratings. Therefore, in controlling the oscillation wavelength, an electrode for adjusting the reflection wavelength and the resonance are used. In order to adjust the phase of the mode, the injection current of both electrodes must be controlled, which has the disadvantage that the control is complicated.

[0004]

Therefore, according to the wavelength tunable device of the present invention, the first super-period grating whose reflection wavelength is variable and the first super-period grating whose reflection wavelength is variable and the center The second wavelength is the same
Quarter-period shift structure in which the super-period grating of the first embodiment is connected to the first super-period grating and the second super-period grating in a state shifted by one-fourth of the period of the grating corresponding to the center wavelength. And a cladding formed on the upper surfaces of the first super-period grating, the second super-period grating, and the quarter-period shift structure, and formed on a portion of the grad that faces the first super-period grating. And a second electrode formed on a portion of the cladding facing the second super-period grating, and on a portion of the cladding facing the quarter-period shift structure. And a third electrode formed to partially cover a portion facing the first super-period grating and the second super-period grating.

In the present invention, a first sample grating having a variable reflection wavelength is provided in place of the first super-periodic grating, and a reflection wavelength is variable in place of the second super-period grating. And a second sample grating having the same center wavelength as that of the first sample grating.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. The drawings used in the description schematically show the dimensions, shapes and arrangements of the components so that the present invention can be understood. In each figure, the same components are denoted by the same reference numerals.

[0007] 1. First Embodiment FIGS. 1 to 4 are views for explaining a first embodiment of the present invention. In particular, FIG. 1 is a perspective view of the wavelength variable element according to the first embodiment, and FIG. 2A is a cross-sectional view taken along line II of FIG. 1 (however, a diagram of an example using super-periodic grating). 2 (B) is an enlarged view of a portion P in FIG. 2 (A), FIG. 3 is a view for explaining the operation, and FIG. 4 is a view corresponding to FIG. 2 when a sample grating is used. .

1 and 2, reference numeral 11 denotes a compound semiconductor substrate, 13 denotes an active waveguide, 15 denotes a first clad,
Reference numeral 17 denotes a second clad, 19a denotes a first electrode, 19b denotes a second electrode, 19c denotes a third electrode, 19d denotes a fourth electrode, and 21 denotes an insulating film. Further, reference numeral 23 denotes a first super-period grating referred to in the present invention, reference numeral 25 denotes a second super-period grating referred to in the present invention, and reference numeral 27 denotes a quarter period shift structure referred to in the present invention.

In this case, the active waveguide 13 is composed of a first waveguide 13a and a second waveguide 13b. The first waveguide 13a functions as a waveguide for adjusting the refractive index, and the second waveguide 13b functions as a waveguide having an amplifying function. Then, the second waveguide 13b, the second
, The first waveguide 13a and the first cladding 15 are laminated on the substrate 11 in this order. However, the second waveguide 13b and the first waveguide 1
3a and the first cladding 15 are both striped layers. Further, the insulating layer 21 is formed on the substrate 11 and on a portion corresponding to the side of the second waveguide 13b.
The thickness is approximately the same as the thickness of the waveguide 13b. The second clad 17 extends over the insulating layer 21.

Also, the first super-periodic grating 23,
In this case, the second super-period grating 25 and the quarter-period shift structure 27
It is formed on the surface of a (details will be described later).

The first to third electrodes 19a to 19c are electrically independent from each other. The first electrode 19a is provided on the first clad 15 and on a portion facing the first super-period grating 23,
The second electrode 19b is on the first clad 15
The third electrode 19 c is provided on the first clad 15 and on the portion facing the quarter-period shift structure 27. However, the third electrode 19c is provided to partially cover the portion facing the first and second super-periodic gratings. The fourth electrode 19d is provided on the back surface of the substrate 11. Then, the second clad 17 is connected to the first to fourth
The ground potential is set so as to function as a common counter electrode for the electrodes 19a to 19d.

Each component in the wavelength tunable element described with reference to FIGS. 1 and 2 can be made of any suitable material according to the design. Although not limited thereto, as a constituent material of the compound semiconductor substrate 11, for example, a first conductivity type (eg, n-type) InP substrate, as a constituent material of the second clad 17, for example, a second conductivity type InP layer, The material of the first clad 15 is a first conductivity type InP layer, and the material of the first and second waveguides 13a and 13b is a predetermined composition (the purpose of the first and second waveguides, respectively). Quaternary material such as InGaAs
Each of the sP layers can be mentioned.

Next, referring mainly to FIG.
The super period gratings 23 and 25 and the quarter period shift structure 27 will be described in more detail.

[0014] The first ultra period grating 23 is repeatedly comprises gradually changing the period of the unevenness (e.g. gradually changed from lambda ₁ to lambda _n) to a portion (a portion that is chirped) with a period X ₁ Configuration. On the other hand, the second
Of the super-periodic grating 25 has a configuration in which a portion where the period of the concavo-convex is gradually changing in the same manner as the first super-periodic grating is repeatedly provided at a period X ₂ (X ₁ ≠ X
₂ ). These first and second super-periodic gratings 2
As shown in FIG. 3, each of 3 and 25 has a characteristic of reflecting light having a wavelength at regular intervals. However, the intervals a and b between the reflection wavelengths are different because the long periods (X ₁ and X ₂ described above) in the super-periodic gratings 23 and 25 are different. Further, since both gratings 23 and 25 have the same degree of change in the unevenness, the center wavelength of both gratings 23 and 25 (for example, the average value of the plurality of reflection wavelengths at the above-mentioned fixed interval when the current injected into the electrode is 0) ) Is the same.

The quarter-period shift structure 27 causes the first and second super-periodic gratings 23 and 25 to divide the period of the grating corresponding to the center wavelength by one-fourth.
They are connected in a staggered state. Here, FIG.
As shown in (B), the first super-periodic grating 2
3 and the uneven portion of the second super-periodic grating 25 have a grating period Λ corresponding to the center wavelength.
_The quarter period structure 27 is configured to be connected in a state shifted by a quarter of _X. However,
Λ _X means that when the center wavelength is expressed as λ _X , λ _X =
Is a period that satisfies 2nΛ _X (Bragg reflection condition).
Note that which part of the two super-periodic gratings 23 and 25 is connected by a quarter-period shift depends on the design. Specifically,: long cycle X _1, when connecting the terminal ends of X ₂ in quarter-period shift structure 27,: Long period X _1, 4 in the middle between the X ₂
When connecting min 1 cycle shift structure 27: at one grating a long period X ₁ or X ₂ end, in the other grating middle of a long period X ₁ or X _2, 4 minutes for one cycle shift structure Any suitable connection structure in the case of connection at 27 is adopted.

Next, the operation of the wavelength tunable element according to the first embodiment will be described. As described above with reference to FIG. 3, each of the first super-period grating 23 and the second super-period grating 25 provided in this wavelength tunable element has a characteristic of reflecting light at wavelengths at regular intervals. In addition, since the longer periods (X ₁ and X ₂ ) of the two gratings 23 and 25 are different, the wavelength interval between peaks is different between the two gratings. When each of the gratings 23 and 25 reflects at the same wavelength (λ ₀ ), the entire waveguide 13 forms a resonator. Since the quarter-period shift structure 27 is provided, the DFB has the same structure as the quarter-shift structure DFB centered at λ ₀ . Therefore, when a semiconductor laser is used, λ
Oscillates at a wavelength of ₀ , and when used as a wavelength filter, the wavelength λ ₀
Light can be selected. Here, the control of the oscillation wavelength (the selected wavelength in the case of a wavelength filter) is performed by controlling the control current It1 supplied to the variable wavelength element via the electrode 19a and the control current supplied to the variable wavelength element via the electrode 19b. Current I
By t2, each sample grating 23, 25
Can be achieved by variably controlling the reflection wavelength. That is, both gratings are controlled so that the wavelength λ ₀ overlaps (see FIG. 3). After that, the phase is finely adjusted by the current Ip supplied to the variable wavelength element via the electrode 19c. Since the quarter period shift structure 27 is provided, the wavelength tuning of both gratings 23 and 25 is performed. Also, since the phase does not change much, the control of Ip can be performed irrespective of the wavelength tuning. Therefore, the wavelength selection control can be performed by controlling the two electrodes, which is easier than the conventional one. Further, the control itself by Ip is unnecessary (the electrode 19
c itself is useless). From these facts, it is possible to obtain a wavelength tunable element using the Vernier scale effect, in which the control of the wavelength selection is easier than before. In the case of the device of this embodiment, the light amplification factor can be adjusted by injecting the current Ia into the second waveguide 13b through the fourth electrode 19d.
This is also convenient.

In the above description, an example using a super-periodic grating has been described. However, instead of the first super-periodic grating 23, a first sample grating whose reflection wavelength is variable is used, and instead of the second super-period grating 25, the first sample grating whose reflection wavelength is variable is used. Even when the second sample grating having the same center wavelength as the grating is used, the same effect as the above-described wavelength variable element can be obtained. An example of the configuration in this case is shown in FIG. 4 using a notation method corresponding to FIG. In this case, the first sample the grating 31 has a repeating comprises configuration of one unit portion irregularities with a period Λ is composed of a part without 31b of portion 31a and irregularities continuous period X _1. On the other hand, the second sample grating 33
Is a unit part irregularities with a period Λ is composed of a portion 33a and the concave-convex portion without 33b continuous and has a repetition comprising a configuration with a period _{X 2 (X 1 ≠ X 2} ). Then, so as to be connected in a state in which the uneven portion and uneven portions of the second sample grating 33 is divided displaced a quarter of Bragg gratings period lambda _X corresponding to the center wavelength of the first sample the grating 31 , A quarter period structure 27 is provided. Also in the case of the element shown in FIG. 4, the first and second sample gratings 31 and 33 are, similarly to the element using the above-described super-period grating, a grating having a characteristic of reflecting light of a wavelength at regular intervals. Become. The intervals between the reflection wavelengths are different because the long periods (X ₁ and X ₂ ) in the sample gratings 31 and 33 are different. In addition, since both the gratings 31 and 33 have the same period of unevenness as that of Λ, the center wavelength of both gratings 31 and 33 (the average value of the plurality of reflection wavelengths at the above-mentioned fixed intervals) is the same. Therefore, this element also operates similarly to the element shown in FIG.

Next, the operation of the wavelength conversion element of the present invention will be further described by using mathematical expressions. The analysis that forms the basis for this explanation is described in Reference II (IEICE Transactions on Electronics) vol.
E76-C, pp.1683-1689 (1993.November)), Document III (Integrated Optics), R. Marz (1994), A
rtech House).

From Document II, the refractive index change Δn of the super-periodic grating due to the grating is expressed by the following equation using a Fourier series.

Δn = n ₁ Re ｛ΣF _k exp [i2 (k
β _s + β ₀ ) z]｝ where n ₁ is a coefficient representing the magnitude of modulation. Also,
F _k is a Fourier transform coefficient. Β ₀ is the average Bragg wavelength. In addition, β _s = 2π / X, where X is the super period of the super period grating (X ₁ or X _{2 in} FIG. ₂ ), or the sample period when the sample grating is used (X _{1 in} FIG. 4). And X ₂ ). From Literature II, the coupling equation between reflected wave R and forward wave S is:

[0021] ^{_{-R '+ aR = ΣixF k *}} exp (i2δ k z) S S' + aS = ΣixF k exp (-i2δ k z) R wherein a is the absorption coefficient, x is the coupling coefficient [delta] _k = beta- _{kβ s -β 0 β = 2n w} π / λ n w: an equivalent refractive index of guided light. From this, k = 1, 2, 3,... Where δ _K = 0.
It is understood that the state is exactly the same as in the case of the normal Bragg reflection wavelength of k = 0 at each wavelength corresponding to. Therefore, the phase is the same as the ordinary Bragg reflection. At the reflection wavelength, the reflected wave is φ = a when δ _k = 0 with respect to the traveling wave.
tan (q _k / δ _k ) → π / 2 (when | xF _k | L → ∞). Therefore, its phase is no different from normal Bragg reflection. The phase by the quarter period shift structure 27 is φ ₄ = πβ / (2β ₀ ), but the reflection peak is β = kβ _s + β from δ _k = 0.
₀ , and φ ₄ = π / 2 + πkβ _s / (2β ₀ ). k = 0 πkβ _s / a is the same phase and (2β ₀₎ = 2πm m: integer _{β s = 4β 0 m 'm} ': the need for an integer = k / m. The above is a description of a quarter-period shift structure between the same super-periodic gratings. Under such conditions, φ ₄ = π / 2 + 2πkm ′, and φ ₄ has an indefiniteness of 2π. Except for this, it depends on k. Therefore, the quarter-period shift structure between different super-period gratings operates similarly to the quarter-period shift structure between the same super-period gratings.

Next, the amplitude of light will be obtained. Assuming that the component of the reflected wave is Bt and the component of the transmitted wave is At, and that the absorption coefficient is a = 0 for simplicity, in the reflection wavelength region, At = AA′exp (iφ ₄ ) + BB ′ ^* exp (−i
φ ₄ ) Bt = B ^* A'exp (iφ ₄ ) + A ^* B ' ^* exp
(-Iφ ₄₎ However, A, B are _{A = cosh (| q k |} L) -iδ k sinh (| q k
_{| L) / | q k |} B = ixF k sinh (| q k | L) / | q k | a is also a ^{_{q k 2 = 4xF k 2 -δ}} k 2. Also,
A ′ and B ′ are δ ₄ ′ and xF _k ′ in A and B, respectively. A ′ and B ′ and A and B are the transmission amplitude and the reflection amplitude at the front and rear stages, respectively. δ _k = δ _k '=
In reflection peak center 0 At = i [cosh (| xF k | L) cosh (| xF
_k ′ | L) + sinh (| xF _k | L) sinh (| x
_{F k '| L)] Bt} = sinh (| xF k | L) cosh (| xF k'
│L) -cosh (│xF _k │L) sinh (│xF
_k ′ | L)]. Therefore, if xF _k and xF _k ′ are almost the same, then each Bragg wavelength (δ _k = δ _k ′ =) as in a normal (uniform grating type) quarter-wavelength shift DFB.
At this time, the light is transmitted (At = i, Bt = 0).

2. Second Embodiment In the above, an example has been described in which the first grating and the second grating have different super periods and sample periods (X ₁ and X ₂ ). However, the present invention relates to a case where the first super-period grating 23 and the second super-periodic grating 25 have the same structure, and a case where the first sample grating 31 and the second sample grating 33 have the same structure. , Respectively. That is, in the configuration described with reference to FIGS. 2 and 4, X ₁ = X
_{The second} case is also applicable. The second embodiment is an example. This will be described below. This will be described with reference to FIGS. Here, FIG. 5 is a perspective view showing the entire device of the second embodiment, FIG. 6 is a cross-sectional view taken along line II-II of FIG. 5 (however, a diagram using a sample grating), and FIG. Is a diagram provided for explanation of operation.

In the configuration described with reference to FIGS. 2 and 4, if only X ₁ and X ₂ are changed so that X ₁ = X ₂ , the oscillation wavelength (the selected wavelength in the case of forming a filter) In the case of the above control, oscillation occurs at an oscillation wavelength corresponding to a plurality of reflection wavelengths, or filtering is performed at a plurality of selected wavelengths. This is because the first and second gratings have the same structure, so that both gratings have the same reflection characteristics. Therefore, in this case, FIGS. 5 and 6
, The first and second super-period gratings (or the first and second sample gratings) are formed as gratings 41 and 43 having the same structure, and these are replaced by a quarter period shift structure 27. In addition to the waveguide 13a in which the gratings 41 and 43 are formed, a waveguide 13x having an equivalent refractive index (refractive index difference Δn) different from the waveguide 13a is further provided. More specifically, in the second embodiment, the second waveguide 13b described in the first embodiment is provided at the position where the second waveguide 13b is provided, instead of the first waveguide 13a. Waveguides 13x having different refractive indexes are provided. Further, in the second embodiment, the waveguide 13x is provided with a third grating 45 for convenience of wavelength tuning (details will be described later).

Next, the operation of the wavelength tunable device according to the second embodiment will be described for better understanding. Each of the first and second sample gratings 41 and 43 having the same structure has a characteristic of reflecting light at wavelengths at regular intervals as shown in the upper part of FIG. Further, as shown in the lower part of FIG. 7, the wavelength selection characteristic of the third grating 45 has a peak at a certain wavelength (however, it is changed to some extent by the fourth electrode). First to third gratings 41, 4
When the light is reflected at the same wavelength (λ ₀ ) at 3, 45, the entire active waveguide 13 including the first waveguide 13a and the waveguide 13x having a different transmission refractive index from the first waveguide 13a. , Constituting a resonator. Further, since the quarter period shift structure is provided, the element of the second embodiment is also the first embodiment.
DF having a quarter shift structure centered on λ ₀ as in the embodiment of FIG.
B has the same structure. Therefore, it oscillates at the wavelength of λ ₀ . Here, the control of the oscillation wavelength can be performed by the control current It3 for controlling the first and second gratings 41 and 43 and the control current It4 for controlling the third grating. After that, fine adjustment of the phase by the current Ip supplied to the wavelength variable element via the electrode 19c is performed. However, since the quarter period shift structure 27 is provided, the two gratings 41, 4
Since the phase does not change much even if the wavelength tuning of No. 3 is performed, the control of Ip is hardly related to the wavelength tuning and the control becomes easy. That is, the control of the oscillation wavelength is controlled by the two electrodes.

In controlling the oscillation wavelength, the second gratings 41 and 43 contribute to fine adjustment of the selected wavelength.
Assuming that the refractive index change due to the control current It3 is δn, the first
The change rate of the selected wavelength in the second gratings 41 and 43 is δn / n. Where n is the first waveguide 1
3a is the transmission refractive index. Further, when the wavelength is λ and the element length of the wavelength selection element is L, the half width of the wavelength selection by the first and second gratings 41 and 43 is λ ² /
(2 nL). Therefore, when L = 500 μm, when the wavelength band to be handled is around 1500 nm, and when the refractive index of the first waveguide is InP, the half-width is about 0.7 nm. On the other hand, the change in the selected wavelength in the third grating 45 depends on the control current I
Assuming that the change in the refractive index of the waveguide 13x due to t4 is δn,
It is about δn / Δn. Here, Δn is the equivalent refractive index difference between the waveguides 13a and 13x as described above. Therefore, δn in the waveguide 13x is 10 ⁻² and Δn is 1
If 0 ⁻¹ , the change of the selected wavelength in the third grating 45 is 100 nm or more. Therefore, also in the second embodiment, the wavelength tunable width is wide and the control during the operation is conventionally performed. A simpler wavelength variable element can be obtained.

[0027]

As is apparent from the above description, according to the wavelength variable element of the present invention, the first variable reflection wavelength can be obtained.
And a second super-period (or sample) whose reflection wavelength is variable and whose center wavelength is the same as that of the first grating.
Since a grating and a predetermined quarter-period shift structure are provided, the wavelength selection control can be performed only by controlling the first and second gratings. For this reason, a wavelength tunable element whose wavelength variable width is at least equal to that of the conventional one and whose wavelength selection control is simpler than the conventional one can be obtained.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram (part 1) of a first embodiment;
FIG. 2 is a perspective view of the element according to the first embodiment.

FIG. 2 is an explanatory diagram (part 2) of the first embodiment;
3A and 3B are a cross-sectional view and a main part explanatory view of the element of the first embodiment.

FIG. 3 is an explanatory diagram (part 3) of the first embodiment;
FIG. 3 is a diagram provided for describing the operation of the element according to the first embodiment.

FIG. 4 is an explanatory view (No. 4) of the first embodiment;
It is sectional drawing and principal part explanatory drawing of an element when a grating is a sample grating.

FIG. 5 is an explanatory view (1) of the second embodiment,
It is a perspective view of the element of a 2nd embodiment.

FIG. 6 is an explanatory view (2) of the second embodiment;
It is sectional drawing and principal part explanatory drawing of the element of 2nd Embodiment.

FIG. 7 is an explanatory view (3) of the second embodiment,
FIG. 14 is a diagram which is used for describing the operation of the element according to the second embodiment.

[Explanation of symbols]

11: Compound semiconductor substrate 13: Active waveguide 13a: First waveguide 13b: Second waveguide 13x: Waveguide having an equivalent refractive index different from that of a waveguide in which a grating is formed 15: First cladding 17 : Second cladding 19a: first electrode 19b: second electrode 19c: third electrode 19d: fourth electrode 21: insulating film 23: first super-period grating 25: second super -period grating 27: 4 minutes One-period shift structure 31: first sample grating 33: second sample grating 41: first super-period (or sample) grating 43: second super-period (or sample) having the same structure as first grating 41 Grating

Claims (2)

(57) [Claims] 1. A first super-period grating whose reflection wavelength is variable, and a second super-period grating whose reflection wavelength is variable and whose center wavelength is the same as that of the first super-period grating. Connecting the first super-period grating and the second super-period grating in a state shifted by a quarter of the period of the grating corresponding to the center wavelength.
A quarter-period shift structure; a clad formed on the upper surfaces of the first super-period grating, the second super-period grating and the quarter-period shift structure; A first electrode formed on a portion facing the super-periodic grating, a second electrode formed on the cladding on a portion facing the second super-period grating, and A third electrode formed on a portion facing the quarter-period shift structure and partially on a portion facing the first super-period grating and the second super-period grating. A wavelength tunable element comprising: 2. The wavelength tunable device according to claim 1, wherein the first super-period grating and the second super-period grating have equal intervals between reflection wavelengths. A second waveguide having an equivalent refractive index different from the equivalent refractive index of the first waveguide forming the super-periodic grating; and a third waveguide formed in the second waveguide and capable of selecting a reflection wavelength.
A wavelength tunable element comprising a grating.