Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2001-02-28

2003-12-09

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

Distributed feedback

C372S045013, C372S046012

Reexamination Certificate

active

06661828

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device which includes a wavelength selecting structure formed in the vicinity of an active layer in a resonator structure, the wavelength selecting structure being capable of selecting a lasing wavelength &lgr;
e
independent of the optical gain distribution of the active layer, for emitting laser light of the selected lasing wavelength &lgr;
e
. In particular, the present invention relates to a semiconductor laser device which is capable of a stable single-mode lasing in a wide temperature range, has a high mode-to-side-mode suppression ratio (SMSR) at the lasing wavelength, and is best suited especially to a light source for optical communication.
2. Description of the Related Art
A distributed feedback semiconductor laser (hereinafter, referred to as a DFB laser) has in its resonator a diffraction grating for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed-back for wavelength selectivity.
In a DFB laser having in the vicinity of its active layer a diffraction grating including a compound semiconductor layer that periodically differs in refractive index from the surroundings, the lasing wavelength &lgr;
DFB
of the DFB laser is determined by the relation &lgr;
DFB
=2n
eff
&Lgr;, where &Lgr; is the period of the diffraction grating and n
eff
is the effective refractive index of the waveguide. Thus, the period &Lgr; of the diffraction grating and the effective refractive index n
eff
of the waveguide can be adjusted to set the lasing wavelength &lgr;
DFB
independent of the peak wavelength of the optical gain of the active layer.
For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution of the active layer, the differential gain increases to improve the DFB laser in high-speed modulation characteristic and the like,
Setting the lasing wavelength of the DFB laser at around the peak wavelength of the optical gain distribution of the active layer decreases the threshold current at room temperature.
Setting the lasing wavelength of the DFB laser at wavelengths longer than the peak wavelength of the optical gain distribution of the active layer makes the temperature characteristic suitable, which improves the operational characteristics at higher temperatures as well as the high intensity output characteristics at higher temperatures or under higher current injection.
By the way, in the conventional DFB laser, the lasing wavelength, whether falling at wavelengths shorter or longer than the peak wavelength of the optical gain distribution, is set within a close wavelength range of several tens of nanometers from the peak wavelength of the optical gain distribution of the active layer. The reasons for this are that (1) the threshold current can be held down, and (2) the single-mode operation is maintained.
Moreover, in the conventional DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the energy of the lasing wavelength. More specifically, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating typically resides in wavelengths 100 nm or more shorter than the lasing wavelength, and accordingly the compound semiconductor layer is transparent to the lasing wavelength, with little light absorption or loss. The diffraction grating which shows periodical, spatial changes in refractive index is fabricated by laminating the compound semiconductor layers, followed by etching to form rows of layers which extend in parallel and periodically.
Here, the conventional DFB laser will be further described in the concrete. The conventional DFB laser can be broadly divided into a first conventional example in which &lgr;e is 1550 nm, &lgr;g falls within the range of 1200 and 1300 nm, and &lgr;g<&lgr;max<&lgr;e holds as shown in FIG.
8
(
a
), and a second conventional example in which &lgr;e is 1550 nm, kg is 1650 nm, and &lgr;max<&lgr;e<&lgr;g holds as shown in FIG.
8
(
b
).
In the first conventional example, &lgr;e−&lgr;g 300 nm. Meanwhile, &lgr;e−&lgr;g=−100 nm in the second conventional example,
Here, the full-lined curve in FIG.
8
(
b
) shows the optical gain distribution of the active layer with respect to the wavelength on the abscissa. The broken-lined curve is a curve showing the amount of absorption (loss) in the diffraction grating layer with respect to the wavelength on the abscissa.
In this connection, &lgr;e is the lasing wavelength of the DFB laser determined by the period of the diffraction grating and the effective refractive index of the waveguide, &lgr;g is the bandgap wavelength of the diffraction grating layer, and &lgr;max is the peak wavelength of the optical gain distribution of the active layer. The bandgap wavelength of the buried layer, or typically an InP layer, in the diffracting grating layer is &lgr;InP (=920 nm).
Nevertheless, in the conventional DFB laser, when the space period of the diffraction grating was adjusted to set the lasing wavelength of the DFB laser at wavelengths longer than the peak wavelength of the optical gain distribution of the active layer, Fabry-Perot lasing sometimes occurred not at the set lasing wavelength of the DFB laser but at the peak wavelength of the optical gain distribution of the active layer.
Moreover, even if the DFB laser lases at the designed lasing wavelength, there is a problem that a side mode suppression ratio (SMSR) of adequate magnitude cannot be secured between the lasing mode at the designed lasing wavelength of the DFB laser and the mode around the peak wavelength of the optical gain distribution of the active layer. For example, in the conventional DFB laser, the side mode suppression ratio (SMSR), though depending on the amount of detuning to the lasing wavelength of the DFB laser, falls within a comparatively small range of 35 and 40 dB. As a result, the conventional DFB laser has a problem in that it was impossible for the lasing wavelength of the DFB laser to be enlarged in the amount of detuning with respect to the peak wavelength of the optical gain distribution of the active layer.
To be more specific, the first conventional example with the lasing wavelength &lgr;e of the DFB laser greater than the bandgap wavelength &lgr;g of the diffraction grating layer has the advantages that the absorption loss at the lasing wavelength &lgr;e is small, the threshold current is accordingly low, and the optical output-injection current characteristics are favorable. However, the smaller difference in refractive index between the diffraction grating layer and the InP buried layer requires a reduction of the distance between the diffraction grating and the active layer. As a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio, which makes it difficult to fabricate same-characteristic DFB lasers with stability.
Moreover, assuming that the absorption coefficient with respect to the lasing wavelength &lgr;e of the DFB laser is &agr;e and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength &lgr;max of the optical gain distribution of the active layer, is &agr;max, then &agr;e ≈&agr;max≈0. This means a smaller suppression effect both in the Fabry-Perot lasing mode in the vicinity of the peak wavelength of the optical gain distribution of the active layer and in the lasing mode of the DFB laser. Accordingly, there was a problem is that the absolute value of the detuning amount |&lgr;e−&lgr;max| cannot be made greater since an increase in the absolute value of the detuning amount |&lgr;e−&lgr;max| lowers the single-mode properties of th

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