JP2000223774A - Wavelength-variable light source - Google Patents

Abstract

PROBLEM TO BE SOLVED: To oscillate at only equally spaced wavelengths to hold the allowance constant for controlling the frequency grid width, by optically coupling a Bragg waveguide with a Machzender waveguide at both sides of an active medium, so that the former has reflected light characteristics having reflection factor peaks at equally spaced frequencies. SOLUTION: A DBR region 2 having a super-periodic structure and a Machzender interference type (MZI) waveguide region 3 are disposed at both sides of an active region 1 of an active medium. A part roughed by etching is planarized with polyimide, the surface of a substrate is patterned by photography to form a p-electrode on the entire surfaces of an SSG-DBR region 2, the active region 1, and a phase adjusting region 4 and a part of the MZI waveguide region 3 and an n-electrode on the entire back surface after polishing, it is cleaved to finish an element, and the period and phase of a diffraction grating on the SSG-DBR region 2 and the super-period are optimized to realize reflection characteristics having equally spaced sharp peaks having approximately the same reflection factor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunable light source. For example, with respect to a multi-wavelength light source, the present invention provides a multi-wavelength light source that can be mainly used as a light source for wavelength division multiplexing (WDM) transmission.

[0002]

2. Description of the Related Art In recent years, in order to cope with an increase in communication capacity,
A WDM transmission system utilizing the broadband characteristics of optical fibers has been put to practical use, and in the future Add-Drop function, Mux-De
A photonic network that performs a mux function with light has been proposed. In the WDM transmission system, light sources having frequencies of 4, 16, 32, and 64 waves arranged at intervals of 100 GHz or 200 GHz are required, and a distributed feedback (DFB) laser is usually used. That is, DFB lasers having different oscillation wavelengths are prepared for the multiplicity.

In a DFB laser, the oscillation wavelength is mainly determined by the period of the diffraction grating.
The B laser must be made separately. In addition, since the oscillation wavelength slightly changes due to variations in the semiconductor film thickness and stripe width at the time of manufacture, deviations in the semiconductor composition, etc.
The yield of the DFB laser that is in the frequency grid used in the transmission method is extremely low, and there is a problem that the manufacturing cost increases.

On the other hand, a wavelength variable light source is very promising as a light source of the WDM transmission system because it can correct the variation of the oscillation wavelength caused by such a manufacturing problem later.
As a semiconductor laser having a wavelength variable function, a distributed Bragg Reflector (DBR) laser is well known. In this technique, a DBR region having a diffraction grating is provided on both sides or one side of an active region, and only a resonance mode near a Bragg wavelength determined by the period of the diffraction grating is selectively oscillated in a single mode. Wavelength selectivity is DBR
It is obtained by changing the Bragg wavelength by a change in the refractive index due to the plasma effect caused by injecting a current into the region.

In this case, the rate of change in wavelength (Δλ / λ) with respect to the oscillation wavelength is equal to the rate of change in refractive index (Δn / n). Since the maximum refractive index change due to the plasma effect of the semiconductor is about 1%, the wavelength variable width is about 10 nm. As a method of increasing the ratio of the wavelength change to the refractive index change, as shown in FIG. 2, the diffraction grating in the DBR region on both sides of the active layer has a super periodic structure (SSG).
A laser (SSG-DBR laser) has been proposed in which a Vernier effect is provided by slightly changing the period of the diffraction gratings on both sides to greatly expand the wavelength tunable range.
A wavelength tunable width close to 100 nm is realized in a quasi-continuous manner.

[0006] In the case of a light source for the WDM transmission system, the wavelength used is usually limited to a certain grid. For example, a frequency grid known in the International Telecommunication Union ITU is:
193.1 THz (1552.52 nm) is used as a reference frequency, and since it is arranged at intervals of 100 GHz therefrom, it is not always necessary to be able to continuously perform wavelength sweeping. S mentioned above
Regarding the SG-DBR laser, the wavelength sweep is realized by injecting current into a total of three regions of the front and rear DBR regions and the phase adjustment region. Because it is realized by a complex combination of parameters, it is hard to find easily.

On the other hand, a DBR laser that selectively oscillates only wavelengths arranged at equal intervals has also been reported. FIG. 3 shows an example in which the longitudinal mode interval Δλ defined by the length of the resonator determined by the length of the active layer and the phase adjustment region is adjusted to the frequency interval, and the fine adjustment is performed by the injection current into the DBR region. In this case, the change in the oscillation wavelength with respect to the injection current into the DBR region is as shown in FIG.

In general, the change in the refractive index due to the current, that is, the carrier injection, is proportional to the square of the injected carriers. Therefore, the change curve of the injection current versus the oscillation wavelength is not linear but close to a parabolic shape. For this reason, the allowable current width for controlling to the frequency grid width differs depending on the wavelength, and the controllability is slightly inferior.

[0009]

SUMMARY OF THE INVENTION In view of the above-mentioned prior art, the present invention is intended to selectively oscillate only equally-spaced wavelengths applicable to a light source for WDM transmission and to control the frequency grid width. Is to provide a wavelength tunable light source having a control method in which the allowable width is constant.

[0010]

A tunable light source according to a first aspect of the present invention for solving the above-mentioned problems comprises a Bragg waveguide and a Mahachenda interference type waveguide optically coupled to both sides of an active medium. Further, the Bragg waveguide has a reflected light characteristic having a maximum reflectance at a plurality of frequencies arranged at equal intervals.

According to a second aspect of the present invention, there is provided a wavelength tunable light source according to the second aspect of the present invention, wherein the active medium, the Bragg waveguide, and the Mach-Zehnder interference type waveguide are monolithically integrated on a semiconductor substrate. It is characterized by becoming.

According to a third aspect of the present invention, there is provided a wavelength tunable light source having a short period in which the period of the diffraction grating of the Bragg waveguide changes continuously. , Characterized by having a super-periodicity in which the short-periodicity is repeated a plurality of times.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to examples. FIG. 1 shows a tunable light source according to the present invention. FIG. 1A is a plan view of a wavelength tunable light source according to an embodiment of the present invention, and FIG. As shown in the figure, a super-periodic structure (Super-Structure Gratin) is provided on both sides of the active region 1, which is the active medium.
g: SSG), and a configuration in which a DBR region 2 having a Mach-Zehnder interference (MZI) waveguide region 3 is arranged.

The DBR region 2 having the SSG structure has a short period in which the period continuously changes and a super-periodicity in which the short period is repeated a plurality of times. Further, a phase adjusting region 4 is inserted between the active region 1 and the Mahachenda interference type waveguide region 3. The phase adjustment region 4 can be omitted. In the present embodiment, the high reflection film 5 is formed on the end face of the Mahachenda interference type waveguide region 3 and the structure is folded, so that the waveguide length is half of the normal length.

The wavelength tunable light source of this embodiment having the above configuration is manufactured as follows. First, an active layer having a multiple quantum well (MQW) structure, a light confinement (SCH) layer sandwiching the active layer, and a part of an upper cladding layer are grown on an n-type InP substrate by a metal organic chemical vapor deposition (MOVPE) method.

The composition of the active layer is adjusted to 1.55 μm.
Set the well width and composition. Next, the final active layer
50 × 300 μm SiN to include the part_x
InP substrate by dry etching using film as a mask
Until it reaches. Continued for the second time
As a growth, an InGaAsP layer having a composition of 1.3 μm was formed on the Si layer.
N _xThe film is selectively grown using the film as a mask.

This layer becomes a waveguide layer of the SSG-DBR region 2, the phase adjustment region 4, and the MZI waveguide region 3, and is Butt-joined to the active layer. After the second growth, a diffraction grating having a super-periodic structure is drawn on a portion to be the SSG-DBR region 2 by an electron beam (EB) exposure method, and is formed by etching. Next, as a third growth, a p-InP layer is grown on the entire surface and a diffraction grating is embedded.

Further, a buried growth step (p
-InP, n-InP, p-InP) are performed on the DBR region 2 and the active region 1 to form a waveguide having a stripe width of about 1.5 μm. As the fifth growth, p-type InGaA
An sP cap layer is grown over the entire surface. Thereafter, the MZI waveguide region is etched so as to form a ridge-type waveguide. The element structure at this stage is as shown in FIG.

Here, after the portion having irregularities formed by etching is flattened with polyimide, the entire surface of the SSG-DBR region 2, the active region 1, the phase adjustment region 4 and the MZI waveguide are formed on the surface side of the substrate by patterning by photolithography. A p-electrode is formed on a part of the region 3 and n is formed on the entire back surface after polishing.
An electrode is formed and cleaved to complete the device.

The operation principle of the construction method according to the present invention is as follows. Here, an example will be described based on the configuration example shown in FIG. FIG. 5A shows the characteristics of the Fabry-Perot (FP) mode determined by the resonator constituted by the cleavage plane.
(B) shows the reflection characteristics of the DBR region 2 having the SSG structure,
FIG. 5C shows the reflection characteristics of the MZI waveguide region 3 as a function of wavelength.

The gain width caused by current injection into the active layer is about 100 nm, and as shown in FIG.
A longitudinal mode appears at every P-mode interval Δλm. For example, if the resonator length is 1.5 mm, Δλm = about 30 GHz. On the other hand, the reflection characteristics of the SSG-DBR region 2 are described in, for example, a literature (IEEE Journal of Quantum Electron).
ics, Vol. 32, No. 3, pp. 433-441, 1996), the period and the phase of the diffraction grating in the SSG-DBR region 2;
It is known that by optimizing the super-period, it is possible to realize a reflection characteristic composed of several sharp maxima (peaks) having equal intervals and substantially the same reflectance as shown in FIG.

The reflection characteristics of the MZI waveguide region 3 are as follows:
The peak position changes due to the change in the refractive index caused by the voltage applied to one of the branched waveguides.
(C). The shift amount of the peak position is MZI
1/1 of FSR (Free Spectral Range) of waveguide region 3
For example, if the optical path length difference ΔL between the two waveguides forming the MZI is 25 μm, the FSR becomes about 60 nm, so that the peak position can be shifted by 30 nm.

The emitted light spectrum is determined by the total reflection characteristics of the DBR region 2 and the MZI waveguide region 3 sandwiching the active region 1, and is applied to the MZI waveguide region 3 as shown in FIG. It can be seen that the voltage changes from λb to λa depending on the applied voltage. Consider the longitudinal mode of the tunable light source according to the present embodiment. Single longitudinal mode oscillation,
Whether multi-mode oscillation occurs depends on the mirror loss difference between the main mode and the sub-mode.

In the case of the wavelength tunable light source of this embodiment, since the oscillation characteristics are determined by the above-described principle, the mirror loss is substantially determined by the reflection characteristics of the SSG-DBR region 2 and the MZI waveguide region 3. The reflection characteristic of the MZI waveguide region 3 is sine-like, and in the case of a configuration in which the FSR is large as described above, the change is gradual with respect to the wavelength.
The oscillation characteristic is mainly determined by the reflection characteristic of the SSG-DBR region 2. On the other hand, the full width at half maximum of each reflection peak in the SSG-DBR region 2 is determined by the number of repetitions m of the short-period, and when m is increased, the full width at half maximum becomes narrower.

Therefore, by controlling m, it is possible to control the longitudinal mode, that is, to perform both multi-mode oscillation and single longitudinal mode oscillation. Generally, the main / sub mode suppression ratio (SMS)
In order to make R) 30 dB, a mirror loss difference of 3 dB is required.
% Or more. The longitudinal mode is affected by the fiber dispersion and defines the transmission distance. Therefore, a single longitudinal mode is indispensable for long distances of about 20 km or more. On the other hand, at short distances, the effect of fiber dispersion is not so effective, so multimode oscillation is sufficient.

In general, a multi-mode laser has higher resistance to return light from the outside, and therefore a multi-mode laser that does not require an isolator for short distances is more suitable. SSG-D for all applications
This can be achieved by optimizing the reflection characteristics of the BR region 2.
FIG. 6 shows how the wavelength changes with respect to the applied voltage of the MZI, and the wavelength shift amount with respect to the change in voltage has a substantially linear relationship.

Therefore, the allowable voltage width for controlling the frequency grid width is constant regardless of the wavelength, and the controllability is high.
Further, when the phase adjustment region 4 is provided, fine adjustment of the wavelength can be performed without depending on the applied voltage of the MZI, and the controllability is further improved. As is clear from comparison with the conventional DBR laser shown in FIG. 4, the tunable light source of the present invention is characterized by having excellent controllability.

[0028]

As described in detail above, according to the present invention, it is possible to selectively oscillate only equally-spaced wavelengths applicable to a light source for WDM transmission and to control the wavelength to a frequency grid width. A tunable light source having a control method with a constant width can be realized.

[Brief description of the drawings]

FIG. 1 relates to a wavelength tunable light source according to an embodiment of the present invention;
FIG. 1A is a plan view (only the waveguide portion is shown), and FIG. 1B is a bird's-eye view (only the semiconductor portion is shown) of the partially cutaway view.

FIG. 2 is a structural diagram of an SSG-DBR laser according to a conventional example.

FIG. 3 is a structural diagram of a DBR laser according to a conventional example.

FIG. 4 is a graph showing a wavelength tunable characteristic of a conventional DBR laser.

FIG. 5 is an explanatory diagram showing the operation principle of the wavelength variable light source according to the present invention.

FIG. 6 is a graph showing a wavelength tunable characteristic of the wavelength tunable light source according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Active area 2 DBR area 3 MZI waveguide area 4 Phase adjustment area 5 High reflection film

Claims (3)

[Claims] 1. A Bragg waveguide and a Mahachenda interference type waveguide are optically coupled to both sides of an active medium, and the Bragg waveguide has a maximum reflectance at a plurality of frequencies arranged at equal intervals. A wavelength tunable light source having reflected light characteristics. 2. The wavelength tunable light source according to claim 1, wherein said active medium, said Bragg waveguide and said Mach-Zehnder interference type waveguide are monolithically integrated on a semiconductor substrate. 3. The Bragg waveguide has a short period in which the period of the diffraction grating changes continuously, and has a super period in which the short period is repeated a plurality of times. Or the tunable light source according to 2.