JP2635649B2 - Distributed feedback semiconductor laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a distributed feedback semiconductor laser that performs optical feedback using a diffraction grating provided along an optical waveguide.

(Prior Art) distributed feedback (DFB: D istributed F eedback) semiconductor laser can be oscillated in a stable single longitudinal mode. Utilizing this property, its position as a light source is being stabilized in the field of optical communication that requires more coherent light. In particular, the reflection at the cleavage end face is non-reflective (AR: Anti-R
eflection) The structure in which the diffraction grating in the optical waveguide is provided with a portion where the phase is discontinuous (phase shifter) is suppressed by a coat or the like, and oscillates near the Bragg wavelength, and at the same time, the longitudinal modes other than the main mode are sufficiently used. Promising because it can be suppressed.

(Problems to be Solved by the Invention) However, the following problems have been pointed out even in a DFB laser having such excellent characteristics. That is, spatial hole burning in the axial direction of the cavity pointed out by, for example, Souda et al., IEICE, Optoelectronics Research Group, OQE86-7, pp. 49-56 (1986).

This is because the light intensity concentrates at one point in the axial direction (the position of the phase shifter in the case of the DFB laser with a phase shifter) in the resonator, and the injected carrier density in this portion decreases. This relative decrease in carrier density causes an increase in the refractive index of the active layer. Due to this change in refractive index,
The guided wavelength of the guided light becomes spatially non-uniform in the axial direction, which changes the structure of the DFB laser. Therefore, the deviation δ (δ) between the threshold gain αth (corresponding to the internal mirror loss) of the DFB laser and the phase constant from the Bragg condition (phase constant: β ₀ )
= Β-β ₀ ) changes every moment. If this continuous change acts in the direction of reducing the sub-mode suppression ratio, the sub-mode suppression ratio becomes zero at the end, and a mode jump to another vertical mode occurs. Therefore, this axial hole burning phenomenon significantly degrades the single longitudinal mode property of the DFB laser.

By the way, in a semiconductor laser, when pulse modulation is performed at a high speed, a phenomenon in which the oscillation wavelength changes with time is known. This phenomenon is called wavelength chirp, and its time-average spectrum is observed with a remarkably wide line width. This phenomenon is also caused by a change in the refractive index with respect to a change in the carrier density, that is, hole burning. In a Fabry-Perot type semiconductor laser using a normal cleavage plane, the wavelength is chirped by spectral hole burning because it is induced by spectral nonuniformity through gain saturation.

On the other hand, in DFB lasers, spatial chirping in the axial direction has a significant effect, so the wavelength chirping phenomenon occurs by a rather complicated mechanism. That is,
DFB lasers have many design parameters,
The degree of chirp varies widely. In some cases, an extremely large chirp can be obtained. The parameters are, for example, the reflectivity of both end faces and the phase thereof,
The shift amount of the phase shifter, the number and position thereof, and the product κL of the coupling coefficient κ and the resonator length L representing the optical feedback amount of the guided light by the diffraction grating, and the like.

When transmitted through an optical fiber having chromatic dispersion,
The transmission pulse waveform is distorted by the equivalent spread of the oscillation line width due to the chirp. For this reason, an element having a large chirp has given a great penalty to long-distance high-speed optical communication.

The present invention is to provide a DFB laser which controls the wavelength chirp occurring at the time of such pulse modulation and has less chirping.

[Means for Solving the Problems] The present invention relates to a distributed feedback semiconductor laser in which optical feedback is performed by a diffraction grating built therein, and a red-shifted chirp of wavelength due to relaxation oscillation in pulse modulation. Is a distributed feedback semiconductor laser having a reflection end face phase or a diffraction grating phase discontinuity set to a predetermined value so as to compensate for the following.

(Operation) The present invention has been made based on the results of intensive studies on elucidation of the mechanism of wavelength chirp in a DFB laser. That is, in the present invention, attention is paid to the fact that the chirp generation mechanism in the distributed feedback laser is established by two or more factors, and the parameters of the DFB laser are set so that the wavelength fluctuations of the lasers cancel each other. For example, if the wavelength chirp due to the factor 1 moves to the long wavelength side (red shift), the adjustment is made so that the chirp due to the factor 2 moves to the short wavelength side (blue shift).

Next, a wavelength chirping mechanism specific to the above-mentioned DFB laser will be described in detail with reference to the drawings.

FIG. 5 is a block diagram showing the mechanism of chirping.

The path a in the figure indicates the causal relationship of chirp due to spectral hole burning seen in a conventional Fabry-Perot laser.

The path b in the figure is caused by the spatial distribution of the carrier density, that is, by the gain saturation due to the reduced interaction between photons and carriers.

In the present invention, the chirp caused by the axial distribution of the refractive index is important. There are two types of paths, a path c that induces a change in the Bragg wavelength itself through a change in the threshold gain αth, and a wavelength shift (△ λ: △ λα−) from the Bragg condition by changing the phase condition δ. There is a path d that directly contributes to chirp via δ).

As described above, in the DFB semiconductor laser, four of a to d are used.
Chirp occurs as a result of different types of pathway mechanisms.

Therefore, when the movement of the wavelength by one of the paths is opposite to the movement of the other paths, the movement of the wavelength is canceled, and as a result, the movement of the wavelength (chirp) can be extremely reduced. That is, the shift toward the short wavelength side (blue shift) and the shift toward the long wavelength side (red shift) can be compensated for each other.

Such compensation can be realized by optimizing the combination of design parameters specific to the DFB laser. The design parameters unique to the DFB laser are the above-described reflectivity and phase of both end faces, the number of phase shifters, the position in the resonator axial direction, the value of κL, and the like.

Hereinafter, the case where the reflection of the end face can be ignored (the reflectance is 3% or less) will be described using the calculation results. The calculation was obtained by solving a rate equation in consideration of the influence of axial hole burning derived by the inventor's earnest research.

FIG. 4 is a schematic diagram of a distributed feedback laser device in the resonator axis direction. This laser device is a BH-type GaInAsP / InP-based semiconductor laser having a cavity length L = 300 μm, and has an oscillation wavelength near 1.554 μm. A diffraction grating 1 is formed on an InP substrate, and a discontinuous portion (phase shifter) 2 of the diffraction grating is formed at the center of the resonator. Also, antireflection (AR) coating is applied to both end surfaces 3, 3 'to reduce the reflectance to 3% or less.

When the semiconductor laser device is fixed to this structure, the product κL of the coupling coefficient κ and the cavity length L and the amount of phase shift △ θ
Is a design parameter that can be changed.

By the way, regarding the light intensity distribution in the resonator axis direction, when a phase shifter of λ / 4 (λ: guide wavelength; that is, △ θ = 0.5π) exists at the center of the resonator, the resonator axis is set to κL = 1.25. It is said that the distribution in the direction becomes the flattest (Sota, et al., Supra). Also, a λ / 8 phase shifter (△ θ
= 0.25π) or 3λ / 8 phase shifter (0.75π), κL = 1.35 is most flattened. Therefore, it can be considered that the light intensity distribution in the axial direction is flattened in the vicinity of κL to 1.3.

In this case, since there is little non-uniformity of the light intensity in the axial direction, spatial hole burning in the axial direction hardly occurs,
No chirp results. That is, the chirp amount is relatively small because of only the effect of spectral hole burning. However, there is red shift chirping (which may be said to be the effect of spectral hole burning) associated with relaxation oscillation at the rise of the pulse. Conversely, at κL〜1.3, the red shift accompanying this relaxation oscillation cannot be excluded. This phenomenon is related to the value of the gain saturation parameter (gain compression factor) ε, and is dependent on the material.

On the other hand, it is well known that the light intensity distribution is depressed at the center of the resonator when the light intensity distribution is small around κL to 1.3, and largely convex at the center when the light intensity distribution is large. Therefore, κL ~ 1.3
Since the relative change in the refractive index due to axial hole burning is reversed around the vicinity, the tendency of chirp is also κL ~
The opposite occurs around 1.3. These κL values are 1.3
If it is larger or smaller, the phase shift amount is adjusted to compensate for the red shift accompanying the relaxation oscillation at the rise of the pulse, unlike in the case where κL is around 1.3, and it is possible to suppress the chirp to an extremely small chirp. However, if the adjustment of the shift amount is erroneously performed, the chirp amount may be increased in reverse. Therefore, it is necessary to set an appropriate shift amount.

In FIG. 3, the time change of the wavelength and the optical output of a distributed feedback laser having a structure in which κL = 1 (<1.3), a phase shift of λ / 8 is provided at the center of the resonator, and both ends are assumed to be non-reflective are calculated. An example is shown. The applied pulse is 100pse for both rising and falling.
c, 500psec width is assumed.

As can be seen from the figure, the light output waveform (a in the figure) and the time variation of the wavelength (chirp) (b in the figure) indicate that when the light output is small (regions I and III), the chirp is inconspicuous. Closely related. In other words, it can be considered that a chirp in a region II having a large light output can be observed in time.

In the figure, the area A indicated by hatching is gain saturation due to spectral hole burning (the mechanism in FIG. 5a).
, The contribution when considering the reduction of the interaction between photons and carriers (gain saturation by another mechanism: corresponding to the mechanism of FIG. 5b), and the region B is the fluctuation of the threshold gain αth (the mechanism of FIG. 5c). ), And the area C shows the contribution due to the variation of the phase constant shift δ (the mechanism in FIG. 5d). It should be noted that the amounts contributing to chirp of each cause are drawn so as to be understood. The top curve X is the sum of the overall chirps. If there is no spatial hole burning, the curve Y is actually the lowest curve.

By the way, if there is no spatial hole burning, red shift (shift to longer wavelength side) (H in the figure) occurs in the region II where chirp is remarkably observable due to relaxation oscillation (spectral hole burning). Next, after the state continues, a red shift (K in the figure) occurs again when the light output falls.

In the laser device having the characteristics shown in FIG.
Since all of the above factors act on the side causing the blue shift, the red shift at the rising edge becomes inconspicuous as shown by the curve X, and the blue shift occurs near the center of the pulse. That is, although the red shift due to the relaxation oscillation is compensated, the amount of the blue shift is too large and the chirp cannot be reduced.

Next, FIG. 2 shows a calculation example of chirp when the value of κL is changed and the phase shift amount at the center of the resonator is λ / 8, λ / 4, 3λ / 8. Each curve shows the overall chirp corresponding to curve X in FIG. Curve a shows the light output.

First, when κL = 1, the wavelength fluctuation amount is small within λ / 4 (0.5
The phase shift amount is closer to 3λ / 8 (0.75π) than that of (π). At λ / 8 (0.25π), the blue shift is too large as described above, which is an opposite effect.

When κL = 2, the red shift is too large in the λ / 8 shift structure, and 3λ / 8 also has a small variation.

Further, when κL = 3, the red shift in the λ / 8 shifter becomes extremely large, so that the wavelength variation in the region II is small at a value larger than λ / 4 and smaller than 3λ / 8.

In the DFB laser, the amount of phase shift △ θ is λ /
When it becomes nπ far apart from 4 (nπ + 0.5π), it oscillates in two longitudinal modes, and the single longitudinal mode characteristic is broken. However, at 3λ / 8 (△ θ = nπ + 0.75π), 0.8 <κL <
In the range of 3.0, △ αL to 0.4 (△ α: threshold gain difference between main mode and submode, L; resonator length), and the single longitudinal mode property is sufficient. If △ αL> 0.2, a single longitudinal mode can be ensured even during high-speed modulation.

As described above, when the reflection at both end faces is small (≦ 3.
0%), the phase shift amount △ θ at the center of the resonator is nπ + 0.5π in the range of 0.8 <κL <3.0 except for the case where κL is around 1.3.
<△ θ <nπ + 0.75π, that is, λ / 4 <△ θ <3λ / 8), it is possible to minimize the fluctuation of the wavelength during pulse modulation without deteriorating the single longitudinal mode.

Although the above description has been given of the case where the diffraction grating is provided with the phase shifter due to the discontinuity of the phase, it can also be realized by controlling the end face reflection phase. In other words, if there is reflection at the end face, the light before and after reflection will feel the diffraction grating phase discontinuously changing due to the relative positional relationship between the reflection surface and the diffraction grating, and equivalently a phase shifter is provided. This produces the same effect as the above. The effect depends on the difference between the amount of light before and after reflection (reflectance) and the relative relationship between the end face position and the diffraction grating phase.

Example A BH-type GaInAsP / InP-based semiconductor laser having a resonator length L = 300 μm and having an oscillation wavelength of about 1.554 μm was prototyped. I
A diffraction grating is formed on the nP substrate, and a discontinuous portion (phase shifter) of the diffraction grating is formed at the center of the resonator. In addition, a non-reflective coating was applied to both end surfaces, and the reflectance was set to 1% or less. The prototype DFB lasers have a 3λ / 8 phase shift at κL〜1 and have a λ / 8 phase shift.

FIG. 1 shows a time-average spectrum when these semiconductor laser elements are modulated by a 1.8 Gbps NRZ pulse. The modulation current is 30 mA and the bias current is 28 mA. The threshold value of both devices is around 20 mA.

As is apparent from FIG. 7A, the half-width of 1 A or less was obtained with the 3λ / 8 shift laser of the present invention, whereas the λ / 8 shift laser was changed to 2 A or more by the blue shift chirp as shown in FIG. A broad spectrum was obtained.

 This result showed good agreement with the calculation result.

[Effects of the Invention] As described above, according to the present invention, the DF having a small wavelength chirp amount is used.
The B laser was obtained with good reproducibility.

[Brief description of the drawings]

FIG. 1 is a spectrum diagram showing a time-average spectrum of the output of a DFB laser according to the present invention and a DFB laser to which the present invention is not applied, and FIG. 2 is a temporal change in wavelength when κL and a shift amount are changed. FIG. 3 is a diagram showing the wavelength change of a DFB laser having a λ / 8 shift structure with κL = 1 according to cause. FIG. 4 is a schematic sectional view showing the structure of a DFB laser with a phase shifter. FIG. 3 is a block diagram showing a causal relationship of wavelength chirp unique to a DFB semiconductor laser. 1 ... Diffraction grating 2 ... Phase shifter (Diffraction grating phase discontinuity) 3,3 '... End face

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazunori Shiraishi 72 Horikawa-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture Toshiba Electronic Device Engineering Co., Ltd.

Claims (2)

(57) [Claims] 1. A distributed feedback semiconductor laser in which optical feedback is provided by a diffraction grating built therein, a reflection set to a predetermined value so as to compensate for a red-shifted chirp of the wavelength due to relaxation oscillation in pulse modulation. A distributed feedback semiconductor laser having an end face phase or a phase discontinuity of the diffraction grating. 2. The distributed feedback semiconductor laser according to claim 1, wherein the reflectivity at both reflection ends is smaller than 3%, and the discontinuity in the phase of the diffraction grating is located at the center of the resonator.
And 0.8 <κL <3.0, except that the product κL of the coupling count κ of light waves by the diffraction grating and the length L of the resonator is around 1.3.
Wherein the phase shift amount Δθ of the discontinuous portion of the central portion is nπ + 0.5π <Δθ <nπ + 0.75π.