JPH0282681A - semiconductor light emitting device - 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] [Summary] The purpose of this invention is to solve the problem of multi-mode or chirping during high-speed modulation in semiconductor lasers for large-capacity, long-distance optical communications, and to develop distributed feedback or distributed reflection semiconductor lasers. The laser has a modulation section and an amplification section that are electrically separated and inject carriers into the active layer, and the length of the modulation section in the active layer is:
It is configured to have a length less than 1/2 of the amplifying section.

[Industrial application field]

The present invention relates to a semiconductor laser for large-capacity, long-distance optical communication. Single-axis mode oscillation is required for semiconductor lasers for large-capacity, long-distance optical communications to avoid deterioration of transmission characteristics due to dispersion of optical fibers.

[Conventional technology]

Distributed feedback (DFB) or distributed reflection (DBR) lasers have conventionally been provided as single-axis mode oscillation semiconductor lasers.

FIG. 3A shows a conventional semiconductor laser structure. In the figure, 10
0 is Nti, 101 is Pti, l 12 is P cladding layer, +13 is active layer, 114 is optical guide layer, 115 is corrugation, 116 is 1 anti, 117 is bias power supply,
The 11th is a pulse oscillator.

Even in these single-axis mode semiconductor lasers, there is a problem that the high-speed timing oscillation wavelength becomes multi-mode, and furthermore, even if single-axis oscillation is performed, the axial mode width increases, which is called chirping.

FIGS. 3B (a) to (c) show input/output characteristic diagrams of conventional lasers.

In the same figure (C), if the laser is biased to I near the threshold value and a rectangular wave electric current m I p is applied to the laser, in principle the optical output of the rectangular wave will be However, in reality, the carrier density in the active layer shown in FIG. ) Overshoots and relaxation oscillation occurs. Therefore, the optical output shown in FIG. 3(a) undergoes relaxation oscillation as the carrier density changes.

This phenomenon occurs because there is a time delay δt between the electric current and the light emission, and when the driving current 2 is applied, the carrier that can obtain a steady optical output P is Density N, more current flows in, carrier density overshoots, and light outputs more than steady value P. Next, due to the recombination of carriers accompanying this optical output, the carrier density decreases by the steady-state value N, and the optical output also decreases accordingly.

In this way, relaxation oscillations occur in carrier density and optical output.

The influence of this relaxation oscillation on the laser characteristics is explained in Section 3C(a).
, (b) will be explained in the figure. Actually, in addition to the main mode, the DFB laser also has several modes with higher thresholds than the main mode, as shown in FIG. 2(a). Therefore, if the peak of the relaxation oscillation is large, light emission in a secondary mode other than the main mode will occur, as shown in Figure (b).
Mode A other than main mode B appears in the emission spectrum.

Furthermore, another effect of relaxation oscillation is Δλ
1, which appears as a broadening of the spectral width.

The carrier density variation ΔN causes the refractive index variation,
When the carrier density increases, the refractive index decreases.As a result, the laser light emission shifts toward the short wavelength side, and the spectrum bulges toward the short wavelength side.

The larger the career fluctuation, the larger the bulge.

As described above, in the conventional structure, due to carrier overshoot, the overall gain increases beyond the threshold gain of other modes, which is larger than the DFB main mode threshold gain, resulting in multimode oscillation. Furthermore, because the refractive index of the active region decreases due to excess carriers due to this overshoot, the DFB main mode itself shifts to the short wavelength side, resulting in a problem that the spectral width increases by Δλ.

[Problem to be solved by the invention]

In conventional DFB laser or DBR laser,
As described above, multi-mode formation or chirping during high-speed modulation is due to temporal and spatial non-uniformity of carrier density in the active region due to high-speed modulation. Therefore, in order to prevent these phenomena and realize stable single-axis mode oscillation even during high-speed modulation, it is necessary to minimize carrier density fluctuations in the active region.The present invention attempts to solve this problem. It is.

[Means to solve the problem]

The present invention provides a distributed feedback or distributed reflection semiconductor laser including a modulation section and an amplification section that are electrically separated and inject carriers into an active layer, and the length of the modulation section in the active layer is: It is an object of the present invention to provide a semiconductor light emitting device whose length is one-half or less of the amplifying section, and to solve the above-mentioned conventional problems when operating as an optical switch and an amplifier.

A principle diagram of the present invention is shown in FIG. 1A.

In FIG. 1A, 12 is a P cladding layer, 13 is an active layer, and 14
is a light guide layer, I5 is a corrugation, and I6 is a substrate. The amplifying section electrodes are composed of 10 N electrodes and 11 P electrodes, while 20. '21 constitutes a modulating section electrode, and the amplifying section electrode 11 and the modulating section electrode 22 are electrically separated. As this electrical isolation method, for example, a high resistance portion is used as shown in 4I in the figure.

17 is an amplifier bias power supply, 18 is a modulation part bias power supply, and 19 is a pulse oscillator.

In the laser of FIG. 1A, the amplifier electrode 1. Let Po be the required optical output when a forward bias is applied to I and the modulator electrode 21. At this time, the current densities injected from the amplifying section electrode It and the modulating section electrode 21 are made to be the same. This state is the same as the conventional example shown in FIG. 3A when a forward bias is applied.

Next, the forward bias of the modulator ti21 is reduced. Po
When is large, the DFB laser continues to oscillate even if the forward bias is zero. At this time, by applying a reverse bias to the modulation section, it can be brought into a non-oscillating state. This is due to the Franz Kjeldysch effect at the time of reverse bias.
This is because loss due to light absorption in the modulation section increases. In this non-oscillating state, a forward bias pulse is applied to the modulation section.

In the case of nine lasers that perform direct modulation, there is an effect of relaxation oscillation, and the optical output increases from the steady state, causing relaxation oscillation.

In the laser of the present invention, the same thing occurs in the modulation section, but
Looking at the whole, there are many carriers in the amplification section, so when looking at the whole, on average, the carrier fluctuations are small.

This situation will be explained using the human output characteristic diagrams shown in FIGS. 18(a) to 18(d). As shown in Figure (d), 1. Even if a rectangular wave drive current of
Relaxation oscillations occur in the optical output in response to changes in carrier density. In other words, even if the drive current is applied in a rectangular shape, there is a delay δt in light emission, so the actual carrier density in the active layer accumulates in the active layer, as shown by diagonal lines in Figure (C), and becomes stationary. When the value N2 is greater than δ, during light emission after δ, the number of carriers becomes smaller than the steady state due to carrier recombination, and relaxation oscillation of the carrier density occurs.

However, in the structure of the present invention, the modulation section and the amplification section are separated, the current density in the surrounding amplification section is constant, and the carrier density is constant, so the active layer when looking at the laser as a whole The average carrier density of is the average value of the amplification section with no fluctuation and the modulation section with fluctuation, and is averaged according to the ratio of the sizes of the modulation section and the amplification section, as shown in the figure (
As shown in b), the fluctuation range becomes smaller accordingly. For example, if the ratio is 2 amplification units: 1 modulation unit, the variation range of carrier density will be 1/3. Therefore, the fluctuation range of the optical output is also shown in the same figure (
It becomes smaller as shown in a).

[Effect]

As described above, according to the present invention, the peak of the relaxation oscillation of carrier density is smaller than that of the prior art, so the following two effects can be obtained.

See FIGS. 1c (a) and (b) ■ As shown in FIG. 1c (b), multimode oscillation during high-speed modulation is prevented.

In the present invention, the peak value of the relaxation oscillation of the carrier density is lower than before, so the threshold gain of other modes having a higher threshold than the fundamental mode is not reached as shown in FIG. No secondary mode oscillation occurs.

(2) As shown in FIG. 6(b), the increase in oscillation spectrum width Δλ during high-speed modulation is reduced compared to the conventional method.

As previously explained with reference to FIG. 3C(b), the variation ΔN in carrier density causes a variation Δn in the refractive index, and as the carrier density increases, the refractive index decreases. Therefore, the laser light emission shifts to the short wavelength side, and a bulge Δλ appears on the short wavelength side of the spectrum. The larger the career fluctuation, the larger the bulge. However, according to the present invention, the variation in the carrier density within the resonator is smaller than in the prior art, so the variation Δλ in the oscillation spectrum width shown in FIG. 1C (b) is reduced.

Next, it will be explained that in the present invention, the length of the modulation section is limited to 172 or less lengths of the amplification section.

The ratio of the amplification section to the modulation section must be considered first from the viewpoint of oscillation stability. In order to obtain stable oscillation during high-speed modulation, the modulation section must be approximately 1/2 or less than the amplification section.

Let's try calculating this.

In a DFB laser, the conditions for maintaining stable busy-mode oscillation are L=0.5 to 1. 0 ---- It is known that the formula is fi1. Here, N: coupling coefficient, L: cavity length.

In order to perform stable single mode operation during modulation, the stability condition of the fi1 equation must be satisfied in both the ON and OFF states of the laser.

[Estimate 1]

LON=1 in ON state. When the upper limit of O is taken as the ratio of the length of the amplification section to 2 and the length of the modulation section to 1, in the OFF state, LOFF = x (2/ (2+ 1) ] L,
N=X(2/3)L,,=0.66, considering the length of the separation layer, LOFF #0. It becomes 5.

That is, it is within the range of Equation 11 above, indicating that stable single mode operation is performed.

Next, we will make a trial calculation for a case where the modulation section is even smaller.

[Estimation 2] When the upper limit of LoN = 4.0 is set for oN-like B, when the length of the amplifier section is 3 and the length of the modulation section is 1, LOFF = x
(3/ (3+ i) t,,.

=X(3/4)=0.75, and even considering the length of the separation layer, LOFF>0.75>. 5 and still fi
It satisfies the stability condition of Equation 1.

As is clear from this estimate, if the amplification section is more than twice as large as the modulation section (that is, the modulation section is 172 times or less than the amplification section), then
This satisfies the stable single mode oscillation condition of the il1 formula.

On the other hand, it is better for the modulation section to be smaller for the following two reasons.

(2) Reducing the dimensions of the modulation section reduces the capacitance, which is advantageous for high-speed operation.

■The smaller the modulation section, the smaller the amount of variation in average carrier density.

From the above, the smaller the modulation part, the better, but if it is made too small, it will not be possible to effectively turn on and off the oscillation, so there is a limit depending on the number of T members needed to turn off the laser oscillation and the size that can be used to form the electrode. be.

[Example] FIG. 2 shows an example in which the present invention is applied to an InGaAsP-based DFB laser. (a) is a plan view, (b) is a sectional view in the direction of the resonator, and (C) is a sectional view perpendicular to the resonator.

In the same figures (a) to (C), 11 is an amplification part electrode, 21
23 is a groove extending to the n-1n PJJ plate 32, which reduces the capacity of the laser and separates the amplifier electrode 11 and the modulator electrode 21.

22 is a groove that does not reach the 1nGaAsP active Ji35 and does not block the optical path (amplifier electrode 11 = modulator electrode 21).
Separate. Further, in order to ensure electrical isolation, a high resistance portion 41 is formed by proton irradiation.

31 is N electrode, 33 is corrugation, 34 is N G
36 is a P-1nP cladding layer, and 37 is a P1nGaAsp contact layer.

Here, the resonator length L9 is 400 μm, the length of the amplifier electrode/I+ is 60 μm, and the length 1t of the modulator electrode is 60 μm2.
Separation part width l. is 20 μm.

The reason why the resonator section is provided in the center in this embodiment is to suppress DFB light emission when each amplifier section is forward biased.

In this example, the threshold value I when the current is uniformly applied to the entire area is 30 mA, and the optical output is 5 mW.
A driving current of 0 mA was required. In this state, 2V is applied to the modulation section.
By applying a reverse bias of , it is possible to make it into a non-emitting state, and by applying a pulse voltage of 3.3 V in the forward direction, pulse oscillation (peak optical output 5 mW) can be obtained, and 5 G b Single axis mode oscillation is obtained during /S modulation, and the axis mode spread due to chirping is also 20 dB.
The down width has been reduced from 0.8nm to 0.4nm.
I was able to narrow it down to

〔Effect of the invention〕

As described above, according to the present invention, carrier fluctuations during high-speed modulation can be effectively suppressed, and stable quaternary-axis oscillation can be achieved. Further, axial mode expansion due to chirping can also be suppressed.

[Brief explanation of the drawing]

Figure 1A is a principle diagram of the present invention, Figures 1B (a) to (d) are input/output characteristic diagrams of the device of the present invention, and Figures 1C (a) and (b> are thresholds of the device of the present invention. Value gain and oscillation spectrum diagram. Figure 2 is a configuration diagram of an embodiment of the present invention, where (a) is a plan view, (b) is a cross-sectional view in the direction of the resonator, and (C) is a cross-sectional view perpendicular to the resonator. Figure 3A is a sectional view of the main part of a conventional laser, and Figure 3B (
A) to (c) are human output characteristic diagrams of the conventional example, and FIGS. 3C (a) and (b) are conventional threshold gain and oscillation spectrum diagrams. 10 is the N electrode 11 is the P electrode I2 is the P craft layer 13 is the active layer 14 is the light guide layer 15 is the corrugation 6 is the substrate 7 is the amplifier section bias power supply 8 is the modulation section bias power supply 9 is the pulse oscillator 0.21 is the modulation section Electrode 2.23 has a groove l.N Electrode 2 has a N- Resistance part 00 is N electrode 01 is Pi4i 12 is 29976 layer 13 is active layer 14 is optical guide layer 15 is corrugation 116 is substrate 117 is bias' @ source 118 is pulse oscillator

Claims (1)

[Claims] A distributed feedback or distributed reflection semiconductor laser has a modulation section and an amplification section that are electrically separated and inject carriers into an active layer, and the length of the modulation section in the active layer is:
A semiconductor light emitting device characterized in that the length is 1/2 or less of the amplifying section.