JP2563196B2 - Coupled distributed feedback semiconductor laser - Google Patents

Description

TECHNICAL FIELD The present invention relates to a semiconductor laser for optical FM used for coherent optical transmission and the like, and particularly to a coupling having three phase changing portions for changing the phase of light by 1/4 wavelength. The present invention relates to a distributed feedback semiconductor laser.

[Conventional technology]

An example of a conventional technique of a semiconductor laser for optical FM used for coherent optical transmission will be described with reference to FIG. Third
In the figure, 11 is an active layer 1 for optical amplification, cladding layers 2 and 3 for confining light, and a diffraction grating 4 for returning the light in a distributed manner to the boundary between the active layer 1 and the cladding layer 2.
And a distributed feedback semiconductor laser (hereinafter referred to as a DFB laser) 12 composed of electrodes 5 and 6, and 12 is an active layer 1, a cladding layer 2 and 3 and a electrode 5 arranged in parallel with the DFB laser 11. It is a semiconductor laser composed of 6 and 6. Here, a constant current is passed through the DFB laser 11 to oscillate in a stable mode. Then, when the injection current of the laser 12 is modulated below a threshold value, the reflection at the right terminal of the DFB laser 11 is modulated by the refractive index modulation of this laser 12, and frequency modulation is applied. .

Another example is shown in FIG. As shown in FIG. 4, this is the active layer 1 and the cladding layers 2, 3
And the diffraction grating 4 for returning the light in a distributed manner
The B laser 13 has a structure in which one electrode 5 is divided into three. The DFB laser having this structure can introduce asymmetry in the refractive index distribution by applying different injection currents to the divided electrodes 5 and can change the oscillation mode. Then, when a bias current of exactly good conditions is applied to each electrode 5 and a modulation current of opposite phase is applied to each of the middle and end electrodes, an FM oscillating in a single mode and having no AM component can be obtained. .

[Problems to be Solved by the Invention]

However, in the structure shown in FIG. 3, the DFB laser 11
There is a problem in that the frequency modulation is applied differently to the phase at the diffraction grating 4, that is, at the end face of the grating, and the modulation characteristics vary among the elements.

On the other hand, in the structure shown in FIG. 4, the oscillation frequency and the modulation characteristics change depending on the operating conditions, so it is necessary to set the condition once, which results in stability during long-term operation and in remote locations. There is a problem in the uniformity of device characteristics.

In view of the above points, the present invention has been made to solve such a problem, and an object thereof is to provide a stable operation for an optical FM that has a small number of adjustment processes of operating conditions and a small variation between elements. It is to provide a semiconductor laser.

[Means for solving the problem]

In order to achieve the above-mentioned object, the present invention divides a DFB laser into four parts, and couples each part between them through a λ / 4 (where λ: wavelength of light) phase shift, and makes each part uniform. The main feature is to add a constant modulation current whose sign is quadrupolarly opposite to that of the normal bias current.

[Action]

Therefore, according to the present invention, the DFB laser is provided with three λ / 4 phase shifts, and a quadrupole modulation current whose sign is reversed in quadrupole is added to each of the four divided parts to obtain a pure current. Frequency modulation (FM) of is obtained. Therefore, it is possible to structurally set the modulation characteristic without setting the modulation characteristic by adjusting the conventional operation condition.

〔Example〕

Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.

FIG. 1 is a basic structural sectional view showing one embodiment of a coupled phase shift type DFB laser according to the present invention. In the figure, 10 ₁ , 10 ₂ , 10 ₃ and 10 ₄ are DFB lasers composed of an active layer 1 for optical amplification, cladding layers 2 and 3 for optical confinement, and a diffraction grating 4, 7,8 and 9 are the phase of light
This is the phase shift part that changes by / 4. As shown in FIG. 1, each of the phase shift parts 7 to 9 has a specific DF
There is a method in which the B lasers 10 ₁ , 10 ₂ , 10 _3, and 10 ₄ are brought into close contact with each other, and the modulation of the active layer width for giving each distributed feedback is configured so that the phase thereof is shifted by a half wavelength. Further, this portion may be formed of a waveguide and the width thereof may be narrower than the width of the active layer. Further, the DFB lasers 10 ₁ , 10 ₂ , 10 ₃ and 10 ₄ are separated from each other to form air, and the optical distance thereof has a phase difference of λ / 4.
It is also possible to adopt a structure in which the edge reflection is not returned to the original. Then, DFB lasers 10 ₁ to 10 ₄ are formed on one of the electrodes 5 are each independently the other electrode 6 is formed in the common. In the drawings, the same reference numerals indicate the same or corresponding parts.

In such a structure, the DFB lasers 10 ₁ , 10 ₂ , 10 ₃ and 10 ₄
When a constant current above the threshold is applied to each of the electrodes 5 and 6 to cause laser oscillation, the oscillation light frequency at that time matches the structurally set Bragg frequency, and stable single-mode operation is performed. Moreover, it has already been clarified in Japanese Patent Application No. 62-104065 relating to the same applicant that the oscillation spectrum width becomes narrow. Here, different modulation currents are further applied to the electrodes 5 and 6. Thus, i-th of each DFB laser _{10 1 ~10 4 (i = I} , II, III, IV) varies the carrier density in the active layer 1, the refractive index change carrier number change in the semiconductor in Therefore, the coupled mode equation that determines the right traveling light wave R and the left traveling light wave S in the laser is Becomes Here, λ is the Bragg wavelength, Δη _i is the refractive index change due to current injection, α _th is the threshold gain, and δ is the deviation of the oscillation frequency from the Bragg frequency. In addition, each DFB
Light waves between laser 10 _1, 10 _2, 10 ₃ and 10 ₄ of the region undergoes a phase shift of lambda / 4. Also, reflection from both end surfaces is eliminated by applying non-reflective coating or the like. When the injection current changes, both the amplitude and the optical frequency of the output light generally change. Here, I of the first to fourth th DFB laser 10 ₁ to 10 ₄ ie I~IV region, IV region and II, III
Current modulation with opposite sign and equal amplitude is applied to the region. This modulation method can be called quadrupole push-pull modulation. Using the above equations (1) and (2) and the boundary conditions for each region, the eigenvalue equation of this system can be obtained. By linearly approximating this equation, α _th , δ when the modulation current is 0
When the deviations Δα _th and Δδ from are calculated, Δα _th −jΔδ = jK2πΔη / λ (3) However, K is a function of α _th and δ when no modulation is applied, and is a function of the product κ L of the feedback amount κ of the DFB laser and the total cavity length L. According to the calculation, K becomes a real number when δ = 0 as in the coupled phase shift DFB laser having the structure of the present invention. Therefore, according to the above equation (3), Δα
_th = 0, Δδ = −K2πΔη / λ. In quadrupole push-pull modulation, the sum of all modulation currents is zero. Therefore, Δα
_{If th} = 0, no AM modulation is received. On the other hand, since Δδ ≠ 0, pure FM modulation is applied.

Table 1 shows the results of similar calculations performed for various current modulation methods other than those described above.
This Table 1 shows the AM when the divided electrodes of the coupled phase shift type DFB laser structure are modulated by various modulation methods.
2 is a table showing how FM modulation is applied.

However, in Table 1, the symbols +, − and 0 indicate the polarities of the modulation currents applied to the respective electrodes. As is clear from the table, the methods a, b and e in the table as the modulation method are pure F
You can get M.

From the above, in order to obtain pure FM
It suffices to operate so that the total modulation current applied to the two outer ones and the total modulation current applied to the two inner ones have the same magnitude and opposite signs. Such a modulation method can be called quadrupole push-pull modulation.

Next, a numerical example of the modulation depth of FM is shown. Δη in the above equation (3) is caused by the modulation Δn of the carrier density,
The relationship between the two is given by Δη = (λα / 2π) a g Δn ...... (4). Here, a _g is the fine powder gain, and α is the line width increasing factor of the semiconductor laser. (3), (4) used, the cure of Δδ frequency variant Delta] f, FM modulation depth Delta] f with respect to light having a frequency f _{Δf / f = (Kλαa g Γ} / 2πη) Δn ...... (5) Becomes Using κL dependent K, shows the result of obtaining κL dependence Δf for 1.55μmI _n G _{a A s} P combined phase shift DFB laser in Figure 2. Here, the above-mentioned Japanese Patent Application No. 62-104065
The .kappa.L which gives the narrowest spectral line width disclosed in No. 2.5 is 2.5. Therefore, when κL is 2.5, Δf
Is about 500MHz when the carrier density modulation is 1%.

In the above description, the lengths of the four parts of the DFB laser divided into four are equal, but pure FM modulation is possible due to the existence of the λ / 4 phase shift. Pure FM can be obtained even if it is slightly different.

〔The invention's effect〕

As described above, according to the present invention, in the coupled DFB semiconductor laser having three phase changing portions for changing the phase of light by 1/4 wavelength, it is possible to reduce the number of phase changing portions by three of the phase changing portions.
Each of the divided parts has an electrode formed independently,
A constant current equal to or higher than a threshold value is applied to each electrode, and a modulation current is further added to this current to obtain the modulation current as described above.
By emitting light with the sum of the modulation currents applied to the outer two of the electrodes and the sum of the modulation currents applied to the inner two being the same, and with the signs reversed, pure FM
Can be obtained. Therefore, the center frequency, the modulation characteristic, etc. are determined when the device having this structure is manufactured, and it is not necessary to adjust the operating conditions during operation. Therefore, there are advantages that long-term operation stability is obtained, variation between elements is small, and uniform element characteristics are obtained when a plurality of elements are used at distant locations.

[Brief description of drawings]

FIG. 1 is a basic structural sectional view showing an embodiment of a coupled phase shift type DFB laser according to the present invention, and FIG. 2 is a quadrupole push-pull modulation 1.55 μm I _n G _a A in the above embodiment.
FIG. 3 shows an example of the .kappa.L dependence of the FM modulation depth of the _s P coupling phase shift type DFB laser, and FIG. 3 and FIG. 4 are structural cross-sectional views of a conventional optical FM modulator. 1 ... Active layer, 2, 3 ... Cladding layer, 4 ... Diffraction grating,
5,6 …… Electrodes, 7,8,9 …… Phase shift part that shifts the optical phase by λ / 4, 10 ₁ , 10 ₂ , 10 ₃ , 10, ₄ DFB laser.

Claims (1)

(57) [Claims] 1. A coupled distributed feedback semiconductor laser having three phase changing portions for changing the phase of light by 1/4 wavelength. The coupled distributed feedback semiconductor laser is divided into four parts by the three phase changing portions and formed independently at each part. A constant current equal to or higher than a threshold value is applied to the four electrodes, a modulation current is further added to this current, and the modulation current applied to the outer two of the four electrodes as the modulation current. A coupled distributed feedback semiconductor laser, wherein the sum and the sum of the modulation currents applied to the inner two have the same magnitude and emit light such that the signs are opposite.