JPH04188687A - Semiconductor laser and its driving method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Fields] The present invention is applicable to technical fields such as communications and recording fields that require modulation of light (laser light, etc.) with little wavelength fluctuation and output fluctuation. The present invention relates to a semiconductor laser suitable for use and a method for driving the same.

[Traditional technology] Conventionally, when modulating, for example, semiconductor laser light with wavelength stability, as shown in Figure 8, a semiconductor laser section (LD) that continuously outputs a constant wavelength with a constant output and a semiconductor laser section (LD) that continuously outputs a constant wavelength with a constant output are used. One method used is to perform optical modulation by dividing the constant output light 81 from the LD into a modulation section (M) that temporally modulates only the output using absorption or other effects. That is, in FIG. 8, there is a wavelength control section in which a grating is formed and the oscillation wavelength is varied by controlling the current injected into the grating (
C), a phase adjustment part (P), and an active part (A) having an active region.
) and a signal 82 that manually inputs a constant output 81 from a distributed reflection type (DBR) laser unit CLD) to a modulation unit (M).
The modulated output 83 is modulated according to the pressure.

There are also examples in which the modulation part (M) is formed integrally with the semiconductor laser part (LD) to output the modulated output 83, and in which the current injected into the active part (A) having an active region is modulated according to the signal. There is also a conventional example in which the output is directly modulated.

C Problems to be Solved by the Invention] However, the above conventional example has the following drawbacks: (1) When the modulation section (M) is not integrated as shown in FIG. ) as L i N b O3
However, it is difficult to optically align the laser section (LD) and the modulation i (M), and the size of the modulation section (M) increases, resulting in an overall increase in size.

(2) When the laser section (LD) and modulation section (M) are integrated Typically, the modulation section (M) has a quantum confinement effect (QC).
Since modulation is performed using changes in the absorption edge due to electric field action, etc., the modulation section (M) generally consists of a wavelength control section (C), a phase control section (P), and an active section (A). The opposite polarity is applied.

Therefore, it is difficult to electrically separate the modulation section (M) and the active section (A), etc., and the ON/OFF ratio of the modulation output becomes insufficient. Since the amount of absorption is large, there are problems such as not being able to obtain a large optical output.

(3) When the active part (A) of the laser (LD) is directly modulated, the phase adjustment part (P) and the wavelength control part (C) are not well connected in phase, causing output and wavelength fluctuations.

This is schematically illustrated as follows.

In the wavelength control section (C), L/nc (in case of 1st order diffraction)
or L/2n, (in the case of second-order diffraction), a change in refractive index is formed with a spatial period (A), where e is the wavelength of light in vacuum, and nc is the effective refractive index, and the wavelength control section By applying an electric field or injecting a current to (C), the effective refractive index nc is changed, and the period (Δ) does not change much, so ncΔ, 2
It is designed to selectively reflect light of this wavelength by changing the wavelength that satisfies the Bragg reflection condition determined by n cΔ or the like.

On the other hand, the refractive index of the active part (A) and the phase adjustment part (P) also changes due to the plasma effect and temperature effect caused by the current injected therein.

Therefore, when oscillating a modulated output by modulating the injection current to the active part (A), the injection current of the phase adjustment part (P) is balanced with the injection current of the active part (A) so as to satisfy the resonance condition. .

This will be explained with reference to FIG. As shown by the arrow (1) in FIG. 7(3), the injection current of the phase adjustment part (P) is set as IMF with respect to the injection current Xaa of the active part (A).
When the phases are matched as shown in 1) (one broken line of the wavelength control section (C) in Fig. 7(1) shows the spatial periodic structure, and the other broken line shows the light wave), the result shown in Fig. 7(3) is As shown by the arrow in (2), the injection current in the active region (A) is +I*-(1-
is the bias component bln is the signal component), then the phase adjustment section (
Since the injected current of P) is kept at IIIF, the phase becomes out of order as shown in (2) of FIG. 7, and the optical output fluctuates. Thus, in the case of direct modulation, fluctuations in optical output and wavelength have been a problem.

Therefore, an object of the present invention is to solve the above problems.

An object of the present invention is to provide a dynamically wavelength-stable semiconductor laser that achieves stabilization of the output wavelength and output level even when the laser is directly modulated, and a method of driving the semiconductor laser.

[Means for Solving the Problems] In the semiconductor laser of the present invention that achieves the above object,
It has two or more current injection regions including the active region, and a small one of these current injection regions ( Both are provided with driving means for synchronizing and modulating the currents injected into the two.

The semiconductor laser may further include a wavelength control section such as a periodic reflection structure provided within the optical resonant path, and may be configured as a dynamically wavelength-stable wavelength tunable semiconductor laser.

Furthermore, in the method for driving a semiconductor laser according to the present invention, the laser is driven so that the total optical path length of the current injection region does not deviate by more than one point during modulation from a preset value due to the injection current.

Hereinafter, according to a typical example, the current injection regions of the present invention will be explained in detail, assuming that the phase adjustment section (P) and the active section (A) are the phase adjustment section (P) and the active section (A), and I@Ps I@A is injected into these as a bias current. At this time, the Io
, ■ Set @＾ (.

Since the injection current rA of the active part (A) is changed according to the modulation signal, the injection current of the phase adjustment part (p) is shifted in the opposite direction in synchronization with this as shown in FIG. Thereby, even during modulation, the total optical path length is prevented from deviating by more than 1 compared to the total optical path length (this is called a set value or predetermined value) when a good output is obtained as described above.

Now, the effective refractive index of each part A and P is n a s n p ,
When the injection region length is LA, Lp, always (predetermined value) - ≦
The current amounts IA and I to each part A and P are changed synchronously so that na La +n, LP≦(predetermined value) 10 is satisfied.

More generally, always (predetermined value) −L≦ΣniL,≦(
It is sufficient to achieve (predetermined value)+λ.
+ is the effective refractive index of the injection region without a periodic refractive index structure or gain (absorption) distribution, etc.).

As shown in FIG. 6, when the current IA injected into the active layer (A) changes as shown, the temperature Ta of synchronous A changes as shown, and the effective refractive index n1 changes accordingly. na La changes.

Therefore, by shifting the injection current (2) of the phase IIIE section P in the opposite direction as shown in the figure, the temperature T of the synchronous P becomes T.

The effective refractive index n, that is, n, L, changes in the opposite direction of
p is changed so as to cancel out the change in na La.

Normally, as the current increases, the temperature rises and n (T) increases, but at the same time, n decreases due to the effect of injected carriers, so
This can be solved by changing the currents I^ and - in synchronization with each other.

More specifically, since Δn (T) is proportional to the gel heat due to ΔI-V, there is no big difference between each part A and P, and the current density per unit length is expressed as J, %J at each part A and P. Then, Δn a kt kΔJ1 Δn p '# kΔJ.

(k is a constant).

On the other hand, 1Δna LAl #lΔn, Lp I (
It is sufficient that the directions of change are opposite to each other (which is sufficient to satisfy the above conditions), so from these relationships, 1
ΔJ,L,IlΔJPL,l, and from now on, regarding temperature change, 11. , l=+1.1 (the increase/decrease in current is reversed in each part A and P).

On the other hand, the change in n due to carrier density is highly structural dependent, but if each part A and P have the same structure, Illll
1Δna (carrier density) L, llΔnp (carrier density) L, I are satisfied with Al=I rspl.

Therefore, the purpose can be achieved by approximately setting l I-al # l l5Pl.

When the structure of each part A%P is different, Ir, AI and lxm
The ratio of pl will deviate from 1.

As for the circuit that synchronizes and modulates the injection currents of each part A and P, a circuit that distributes the same current used in high-speed modulation of the LD to each part may be used.

In the above principle of the present invention, the modulation speed of the modulation current is I
Although it is particularly effective in the region of about MHz to I GHz, it is also effective even below 1 MHz when the laser according to the present invention having a DBR structure is used in an optical wavelength division multiplexing communication system.

Temperature changes do not pose much of a problem in the region above IGHz, so the problem addressed by the present invention is not so much of a problem, but the concept of the present invention is effective even in this region when the duty fluctuates.

In addition, in the above, the reason why it is necessary to prevent the total optical path length from shifting by more than 1 is because if the total optical path length shifts by more than This is because the degree of instability becomes difficult to tolerate.

[Embodiment J FIG. 1 is a diagram schematically showing a first embodiment of the present invention. In the figure, as mentioned above, C, P, and A are the wavelength control section, phase adjustment section, and active section, respectively (hereinafter, C section, P section, and A section, respectively).
The A part and the P part are made of n-GaAs on an n-GaAs substrate 1 by the MBE (molecular beam epitaxy) method.
Buffer (thickness ILLm, carrier concentration lXl0"c
m-1), n-A Lo, s Gao, s As (1
, 5 parts m, 5xl O"cm""), non-doped (φ)
A 1 o, + G aa, s AS active layer 2 (0
,08um)-p-A1o,sGa
o,s As (1.5gm, 7X 10"cm-")
, p-GaAs (0.5 μm, lXl0"cm-")
It has a layered structure grown by: In part 6, after growing the layer as described above, etching is performed until the φ-AIGaAs active layer 2 is removed, and a periodic grating 3 with a period of 0.22 μm is formed thereon by two-beam interference exposure method, and then MOCVD (organic p -Alo, s Gao, s by metal pyrolysis) method
As and p-GaAs were regrown to the same thickness as above.

Next, in order to fabricate a waveguide structure with lateral confinement, the GaAS layer near the substrate 1 is etched to a width of about 3 μm, and the high resistance A l u, a G is etched again using the MOCVD method.
aa, y A S was embedded.

Electrodes 4.5, 6 and a common electrode 9 are formed in the 6th part, the P part, and the A part, respectively, and the current injection region length is 500 am in the 6th part.
, the P part is 30 pum, and the A part is 300 μm. Each part C
, P, and A are etched by etching approximately lum from the surface to form grooves 7.8 between C and P, and between P and A, in order to avoid crosstalk due to mutual injection currents. Current injection into the three regions of the n-type substrate l and p-type is performed by the ohmic electrodes 4, 5, 6, and 9, respectively.

In the above configuration, 100mA in section 6 and 60mA in section A.
When 40mA is injected into A and P parts, one oscillation wavelength is 820nm.
An output of 2 mW was obtained from the end face of part A.

Next, section A is 200MHz RZ (return to zero)
The signal was modulated by the injection current. That is, 2.5nse in part A
90 mA was applied with C pulse (!I! As shown in Figure 12 (1), the bias component 1.A of the injection current ■ to part A was set to 60 mA, and the signal component I m was applied with 2-5 nsec C pulse.
A is set to 30 mA), and at this time, the injection current I to the second part is
When the ap was left at 40 mA, the output P Out fluctuated as shown in Figure V42 (1). To overcome this, it is necessary to synchronize with the injection current In to part A (see Fig. 2).
As shown in 2), when the current I injected into the P section is reduced in the opposite direction (direction of decreasing from the bias component), (I
As shown in FIG. 2 (2), the optical output P out became stable. That is, the zero of the RZ signal (
At the time of 0), the optical output P orr becomes 2 mW, and at the time of "1'", the optical output P orr becomes 2 mW. , l are 5 mW, and POo has a stable output.

When the device shown in Figure 1 is used as a wavelength tunable laser and the current injected into the 6th section is changed from 100mA to 30mA, 6
The Bragg wavelength (n cΔ) of the part changes, and the oscillation wavelength becomes 8
It becomes 821 nm from 20 nm, and the injection current IA of part A =
The injection current of the A part that obtained an output of 10 mW at 80 mA and the injection current Ip of the P part = 30 mA is Ima + Isa =
80mA, IsA=50mA (i.e. I m &= 30
When the 100MHz RZ signal is modulated with mA), if the injection current I2 of the P section is kept constant, IA = 80m
Light output P0 at time A. fluctuated between 10 mW and 3 mW (see P aul in Figure 2 (1)) a I a =
At 50 mA, P and lJt were 0.1 mW.

Therefore, as the injection current 11 in the A section changes from 80 mA to 50 mA, the injection current 11 in the P section is changed to 30 mA.
A CI -= 80 mA), 60mA (
1, = 50 mA), stable modulated outputs of 10 mW and 0.1 mW of ON and OFF optical outputs were realized.

The same tendency as in the first example was observed in the second example, which had the same structure as the example shown in FIG. 1 but had a length of P portion of 200 μm.

That is, for the wavelength set in the C part, the bias component 1. of the injection current in the A part. ,, After determining the bias component I of the injection current of the P section mp to a constant value, the signal component I of the injection current of the A section. When the signal component of the injection current Imp of the P section was set to 2 s m A (in the direction of decrease) for 30 mA, the optical output P was stabilized.

Next, a third embodiment will be described. In the third embodiment, the P part,
Both portions C were etched including the non-doped AlGaAs active layer, a periodic grating was formed only in the portion C, and a waveguide injection region was formed using the same embedding process as in the first embodiment. That is, in this embodiment, the laminated structures of the P section and the A section are different.

In this case as well, the C section is 100 mA and the A section is 60 mA.

With an injection current of 50 mA in the P section, an optical output of 2 mW at 820 nm was obtained. However, in order to perform 200MHz modulation, the injection current of part A must be 90mA (=I *** I s
When a), the injection current of P part is 40mA (=■5p
(=so)+IIF(=-10)). That is, for Isa=30mA, Isp
It was necessary to reverse the direction at =-10mA. In this third embodiment, the layer structure of the A section and the P section is different, and the plasma effect due to the injection current is different, and the degree of refractive index change is different, so the absolute values of 11A and ■sp are as shown above. It seems different.

Of course, the present invention can also be applied to the case where optical output with a stable wavelength and stable level is obtained by direct modulation without wavelength tuning. This corresponds to the case where Ie (current injected into the C portion)=0 in FIG. 1, and the case where there is no injection current in the C portion as shown in FIG.

Further, the P part and the A part are relative, and even if the arrangement with respect to the C part changes, the intended effects of the present invention will still be produced.

The materials are not limited to GaAs with an oscillation wavelength of around 0.8 μm, but also include InP with an oscillation wavelength of around 0.8 μm, InP with an oscillation wavelength of around 1.5 μm, AlGa1nP with a wavelength of around 0.7 μm, and Zn with an even shorter wavelength.
5e, ZnS, etc. can also be used.

Furthermore, the wavelength determining part, which is the C part, does not need to be integrated, and the C part may be separated from the P part and the A part. An example of this is shown in FIG. The external resonator is rotated to change the oscillation wavelength.

Furthermore, the idea of the present invention is effective even when copies other than 0 are divided into three or more parts as shown in Figure 5 (in the case of Figure 5,
In such a case, it is sufficient to synchronize and vary the currents in the two injection regions, so there is no need to vary one injection region.

Information such as Ic, Ip, ①, etc. for the oscillation wavelengths described in the explanations from the first embodiment to the third embodiment is RO
M, and when selecting the necessary oscillation wavelength, the C, P, and A parts are automatically controlled (this method is also possible. At this time, the l1lPZIsa ratio etc. are also controlled by the P part, A part It is possible to select an incorrect (signal level) by storing it as a value specific to the structure of the signal.

[Effects of the Invention] As explained above, according to the present invention, the injection currents into two or more current injection regions are synchronously modulated so that the total optical path length of these regions does not deviate by more than one from the set value. Because it is
It has become possible to stabilize the output wavelength and output level of semiconductor lasers.

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram of the first embodiment of the present invention, Fig. 2 is a diagram showing the relationship between fluctuations in current injected into each part of the first embodiment and changes in optical output, and Fig. 3 is a diagram showing the relationship between changes in optical output and wavelength tuning. FIG. 4 is a schematic diagram of an example of an external resonator; FIG. 5 is a schematic diagram of an example of an example in which there are three or more parts other than the 0 part; FIG. 6 is a diagram of the driving method of the present invention. FIG. 7 is a diagram explaining a conventional ff problem, and FIG. 8 is a diagram explaining a conventional example.

Claims (1)

[Claims] 1. A semiconductor laser having two or more current injection regions including an active region, wherein the total optical path length of the current injection regions deviates by more than λ from a preset value by the injection current during modulation. 1. A semiconductor laser comprising driving means for synchronizing and modulating the injection current to at least two of the current injection regions so that the current injection region does not change. 2. The semiconductor laser according to claim 1, further comprising a wavelength control section so as to be wavelength tunable. 3. The driving means synchronously modulates the injection current so that when increasing the injection current to a certain current injection region, the injection current to another current injection region is synchronously decreased. semiconductor laser. 4. The semiconductor laser according to claim 2, wherein the wavelength control section is a periodic reflection structure provided within an optical resonant path. 5. The semiconductor laser according to claim 2, wherein the wavelength control section is a periodic reflection structure provided outside the optical resonance path. 6. The semiconductor laser according to claim 3, wherein the ratio of increase and decrease of the injection current is set depending on the structure of each current injection region. 7. The semiconductor laser according to claim 3, wherein the amounts of increase and decrease in the injection current are approximately equal. 8. The method of driving a semiconductor laser according to claim 1, 2, 3, 4, 5, 6, or 7, wherein the total optical path length of the current injection region does not deviate by more than λ from a preset value by the injection current during modulation. A driving method characterized by driving.