JPH0671119B2 - Optical filter element - Google Patents

Description

The present invention relates to an optical filter element, and more particularly to an optical filter element having a distributed feedback structure.

[Conventional technology]

An optical filter element having a function of selecting an arbitrary optical signal from a wavelength-multiplexed optical signal is one of key devices applicable to a wide range of applications in optical transmission, optical switching, optical information processing and the like. In any application, sufficient wavelength selectivity and variable tuning width with a wide selection wavelength are required as the characteristics of the optical filter element. Further, since an optical integrated circuit is indispensable as a structure, it is also necessary to be a transmission type wavelength selection filter that transmits only a desired wavelength to be selected.

BACKGROUND ART Heretofore, some studies have been made on transmissive wavelength selection filters. Among them, an optical filter element having a structure in which an optical feedback structure having wavelength selectivity is provided in an optical amplification element using a semiconductor active layer is capable of tunable selection wavelength depending on the concentration of carriers injected into the active layer. It is also expected to be suitable for transmissive integration. In particular, as an optical feedback structure, a distributed feedback structure composed of a diffraction grating is more advantageous than a Fabry-Perot resonator by cleavage in terms of wavelength selectivity and optical integration, and an optical filter element using these structures is provided. Has also been proposed and theoretically examined [Optics Communications (Optics C
ommunications) Vol. 10, p. 120).

[Problems to be solved by the invention]

However, the optical filter element having a distributed feedback structure in the optical amplification element which has been proposed and studied so far, has the same effect on the selected wavelength when the carrier concentration injected into the active layer is adjusted for tuning the selected wavelength. Since the optical gain and the intensity of spontaneous emission also change, the carrier concentration injected into the active layer is theoretically limited in order to obtain the wavelength selectivity required for the optical filter element and the noise signal intensity ratio. As a result, since the variable tuning width of the selected wavelength is as small as several Å, it has the drawback that it can only be used as a filter for several channels.

An object of the present invention is to provide an optical filter device having several tens of channels or more, which has an amplifying function and eliminates the above-mentioned drawbacks and which can obtain a large variable tuning width of a selected wavelength.

[Means for solving problems]

In the optical filter device of the present invention, a plurality of distributed feedback regions having a phase shift structure and a plurality of active regions are arranged in series, and both end faces of the device are optically coupled and have a non-reflection structure. It is characterized by that.

[Action]

The optical transmission characteristics of an optical waveguide having a distributed feedback structure are determined by the Bragg wavelength determined from the optical period of the diffraction grating when the optical phase of each distributed feedback region is aligned when it has no optical gain in the transmission wavelength range. A single transmission blocking wavelength region of about several tens of liters is formed around. On the other hand, when the optical phase is deviated by π on both sides of the center of the distributed feedback region (In this case, if the wavelength of the light propagating in the element is λ, the pitch of the diffraction grating is deviated by λ / 4. Corresponds to
/ 4 shift structure), the transmission blocking wavelength region is split, and 1 to 2 is formed in the center of the above-mentioned transmission blocking wavelength region of about several tens of liters.
A narrow transmission wavelength range of about Å or less occurs. If the optical waveguide layer having the distributed feedback region having the λ / 4 shift structure is made of a semiconductor having a composition shorter than the transmission wavelength, there is no optical gain. Therefore, about 1 to 2 Å or less centered on the Bragg wavelength by carrier injection. A wide wavelength selection range can be obtained with a narrow width, and there is no deterioration of the noise intensity ratio due to spontaneous emission light.

Further, the amount of phase shift can be adjusted by injecting current into the phase shift region from the outside. Depending on the amount of phase shift, the transmission characteristic changes as shown in FIG. 3 (which shows the transmission characteristic of the optical filter element when the amount of phase shift is changed).
A tunable transmissive optical filter element capable of selecting an arbitrary wavelength in a wavelength range of several Å to several tens of Å centered on the Bragg wavelength is obtained. That is, by making the amount of phase shift variable, it is possible to further increase the variable wavelength width as compared with the case where the amount of phase shift is fixed to only λ / 4.

Since the above-mentioned optical waveguide layer has no gain, the active region and the distributed feedback region may be optically coupled to each other in order to form an optical filter element having an amplifying function. That is, if the structure is such that light is injected into the active region, amplified, and then transmitted to the distributed feedback region, an optical filter element having an amplification function can be obtained.

There is one point to note here. That is, both end faces of the optical filter element must have a non-reflection structure. The reason for this is that if it does not have a non-reflective structure, it will oscillate as a DBR (distributed feedback) laser.

Now, as a feature of the present invention, a plurality of distributed feedback (DBR) regions are arranged in series in the present invention. If there is a single distributed feedback region, the wavelength selection width will be limited to half the transmission blocking wavelength region. However, as in the present invention, by providing a plurality of distributed feedback regions and adjusting the transmission blocking wavelength band of the second distributed feedback region to the wavelength region that could not be blocked by only one distributed feedback region, the wavelength selection width can be increased. Can be large.

〔Example〕

 Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing the structure of an optical filter element according to an embodiment of the present invention. The structure of this embodiment will be described below according to the manufacturing procedure. First, a λ / 4 shift diffraction grating having a period of 2400Å is formed in the distributed feedback (DBR) regions 201 and 202 on the n-type InP substrate 110. Next, by the first LPE growth (liquid phase growth), the undoped InGaAsP optical guide layer 120 (λg =
1.3 μm, thickness 0.3 μm), n-type InP buffer layer 130 (thickness 0.1
μm), Nin-doped active layer 140 (λg = 1.53 μm, thickness 0.1
μm) and a p-type InP cladding layer 150 (thickness 0.2 μm) are sequentially grown. Next, the InP clad layer 150 in the distributed feedback regions 201 and 202
And the active layer 140 are selectively removed. Next, the p-type InP cladding layer 160 is formed on the entire surface by the second LPE growth.

Next, after performing mesa etching to form a buried structure, the buried growth is performed by the third LPE growth. Here, a double channel planar buried structure is used as the buried structure. Finally, after forming electrodes on the substrate side and the growth layer side, a groove having a width of 20 μm is formed except for the vicinity of the central mesa in order to electrically separate the amplification regions 100, 101 and the distributed feedback regions 201, 202. After that, a SiN film 170 is formed by using a plasma CVD apparatus in order to reduce the reflectance of both end faces of the element to 1% or less. The lengths of the amplification regions 101 and 102 and the distributed feedback regions 201 and 202 are 100 μm, 100 μm, 500 μm, and
It is 500 μm.

An example of the transmission characteristics of the optical filter element of the present embodiment manufactured as described above will be described with reference to FIG. The extinction ratio is 20 dB when a current of 50 mA is injected into the amplification regions 101 and 102.
As mentioned above, the 10 dB attenuation width of the transmission wavelength was 0.5 Å. Also,
As shown in FIG. 2, when no current is applied to the distributed feedback regions 201 and 202, the transmission wavelength is 1.5563 μm, and when a current of 80 mA is injected, the transmission wavelength is 1.5505 μm, and the transmission wavelength is continuously 58Å. changed.

When there is only one distributed feedback region, the width of the selected wavelength is limited to half of the transmission blocking wavelength region due to the transmission characteristics, resulting in about 30Å, but if there are two distributed feedback regions, each Bragg By appropriately adjusting the wavelength, the width of the selected wavelength can be doubled to the variable wavelength upper limit of 58Å. This is shown in FIGS. 4 and 5. FIG. 4 is a diagram showing the relationship between the input spectrum and the output spectrum when there is only one distributed feedback region, and FIG. 5 shows the relationship between the input spectrum and the output spectrum when there are two distributed feedback regions. FIG. As shown in FIG. 4, since light having a wavelength in the range A shown in the figure is also transmitted, the wavelength selection width is limited to half the transmission blocking wavelength range. However, when there are two distributed feedback regions, the wavelength selection width can be increased as shown in FIG. This measured value is 58Å, which is 10% compared to the number of wavelength selection widths Å of the conventional optical filter element.
It is possible to double the size.

As can be seen from the above, the optical filter element of the present invention enables arbitrary wavelength selection from signals wavelength-multiplexed within the range of 58Å at intervals of about 0.5Å. I.e. 100
It can be used as a wavelength tunable filter for more than channels.

Although two distributed feedback regions are provided in the above embodiments, the number of distributed feedback regions is not limited to two, and the number of distributed feedback regions may be three or more. Also, the number of amplification regions may be three or more. Furthermore, the material and composition of the element are not limited to those in the above-mentioned embodiments, but other semiconductor materials or dielectric materials may be used. Further, if the phase shift structure is also configured to control the amount of phase shift from the outside, the wavelength tunable range can be further expanded. Also,
In the above-mentioned embodiment, the phase shift diffraction grating is used as the phase shift structure, but it is not limited to the phase shift diffraction grating and has a uniform diffraction grating and the width or thickness of the waveguide is changed. It may be a so-called equivalent phase shift structure. Further, the non-reflective structure may be a window structure or a multilayer film coat.

〔The invention's effect〕

Although the conventional optical filter element has a limit of several channels, the optical filter element of the present invention enables arbitrary wavelength selection from wavelength-multiplexed optical signals of 100 channels or more.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of an optical filter element of an embodiment of the present invention, FIG. 2 is a diagram showing the transmission characteristics of the optical filter element of the embodiment of FIG. 1, and FIG. FIG. 4 is a diagram showing the transmission characteristics of the optical filter element when the phase shift amount is changed, FIG. 4 is a diagram showing the relationship between the input spectrum and the output spectrum when there is only one distributed feedback region, and FIG. It is a figure which shows the relationship between an input spectrum and an output spectrum when there are two distributed feedback areas. 101,102 …… Amplification region 110 …… Substrate 120 …… Optical guide layer 130 …… Buffer layer 140 …… Active layer 150,160 …… Clad layer 170 …… SiN film 201,202 …… Distributed feedback region

Claims (1)

[Claims] 1. A device comprising a plurality of distributed feedback regions having a phase shift structure and a plurality of active regions arranged in series and optically coupled to each other, wherein both end faces have a non-reflection structure. Optical filter element.