JP3264652B2 - Optical circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circuit having an optical waveguide disposed on a flat substrate, and particularly to a wavelength multiplexing / demultiplexing optical circuit, which is useful in the fields of optical communication, optical information processing, and optical measurement.

[0002]

2. Description of the Related Art Quartz optical waveguides have low loss and
It is characterized by high stability and reliability based on the physical and chemical properties of quartz. Further, it has characteristics such as good matching with the fiber because it has almost the same refractive index value as the silica-based fiber which is a transmission line of optical communication. Optical circuits such as optical wavelength multiplexing / demultiplexing devices and optical switches fabricated on a flat substrate using this silica-based optical waveguide are practically promising optical communication, optical information processing, and optical measurement fields. Research and development are progressing as corrugated optical components.

A quartz optical waveguide is generally manufactured as follows. That is, a lower cladding film layer is deposited on a substrate, a core film layer deposition and a core partial patterning process are performed, and an upper cladding layer is further deposited to produce an optical waveguide. Research and development on various optical circuits that are promising for practical use, such as optical power splitters, wavelength multiplexers / demultiplexers, filters, and switches, using this silica-based optical waveguide are being conducted.
(M. Kawachi, "Silica waveguides on silicon and the
ir application to integrated-optic components ", Op
t. and Quantum Electron., 22, pp. 319-416).

[0004] In particular, wavelength multiplexing / demultiplexing optical circuits such as an arrayed waveguide type wavelength multiplexing / demultiplexing device (AWG) have recently been developed with the practical use of wavelength division multiplexing (WDM) optical communication systems. It is becoming a very important optical device. Since the Si substrate has high thermal conductivity, it is easy to uniformly control the substrate temperature, and has the advantage that a highly accurate and smooth substrate can be obtained relatively cheaply and easily. . Therefore, in the production of these quartz-based optical circuits, a Si substrate is often used as the substrate. However, a quartz-based waveguide fabricated on a Si substrate usually has birefringence due to a difference in thermal expansion coefficient between the substrate and the glass layer. This birefringence causes polarization dependence of a transmission blocking region in a wavelength multiplexing / demultiplexing optical circuit or the like, and causes polarization-dependent loss (Polarization Dependent Loss: P).
DL). Since PDL causes deterioration of communication quality, efforts have been made to reduce or compensate for this polarization dependence.

[0005] For example, a method of reducing birefringence by utilizing polarization dependence (anisotropic) of refractive index change Δn induced by irradiation of ultraviolet laser light (M. ABE, et al., “Photoinduced
Birefringence Control in Arrayed-waveguide Gratin
g Multi / Demultiplexer ", MOC'93, pp.66-69, 1993.)
And a method to control birefringence by loading amorphous silicon film (H. Takahashi, et al., "Polarization-Insensi
tive Arrayed-WaveguideWavelength Multiplexer with
Birefringence Compensating Film ", Photon.Technol.
Lett. Vol.5, No.6, pp43-45, 1993). Also, a method of adjusting the composition of the core portion and the cladding portion to reduce birefringence (for example, see S. Suzuki, et al., "Pol
arization-Insensitive Arrayed-Waveguide gratings U
sing Dopant-Rich Silica Based Glass with Thermal E
xpansion Adjustedto Si Substrate ", Electron. Lett.
33, 13, pp. 1173-1174, 1997.).

In addition, a half-wave plate is inserted into an array waveguide portion of an array waveguide type wavelength multiplexer / demultiplexer (AWG), which is a typical wavelength multiplexer / demultiplexer optical circuit, and the transmission blocking wavelength characteristic is obtained. (For example, Y. Inou
e, et al., "Polarization Mode Converter With Polyi
mide Half Waveplate in Silica-Based Planar Lightwa
ve Circuits ", Photon. Technol. Lett. vol.6, No.5,
pp. 46-48, 1994.) are also known, and certain good characteristics are obtained.

However, with the advancement of networks, A
It has been proposed that a wavelength multiplexing / demultiplexing optical circuit such as a WG be used not only for simple wavelength division multiplexing of signal light but also for wavelength monitoring and control of the signal light, and studies are underway. In such applications, the "tail" of the transmission region is used instead of the relatively flat transmittance near the center of the transmission region (M. Teshima, et al., "Performance of Mult
iwavelength Simultaneous Monitoring Circuit Employ
ing Arrayed-Waveguide Grating ", J. LightwaveTechno
l., Vol.14, No.10, pp.2277-2285., 1996).

FIG. 1 is a graph showing a relationship between a transmission spectrum of an AWG optical circuit and a monitoring light wavelength corresponding thereto. In the figure, the relative optical frequency corresponding to the monitoring wavelength, and the first output port (port 1) and the second output port (port
Focusing on the transmission spectrum of 2), the relative optical frequency (GHz)
And the transmittance (dB). As shown in the figure, in the “foot” of the transmission area, a slight difference in the transmission area shows a large PD.
Therefore, there is a demand for an optical circuit having an extremely low PDL that has reduced or compensated the polarization dependency with higher precision than ever before.

[0009]

However, since the attempt to nullify the birefringence by adjusting the composition of the waveguide is performed in addition to the control of a small relative refractive index difference (Δ), it is very delicate and difficult. Adjustment is required, which causes a decrease in the yield of optical circuit production. On the other hand, in order to compensate for the polarization dependence by simply inserting a wave plate, the optical circuit must be completely symmetrical, and its compensating ability depends on the accuracy of the wave plate itself and the accuracy of the wave plate insertion position. There is a limit, and there is a problem that it is difficult to manufacture the above-described optical circuit in which the polarization dependency is compensated with high precision with high yield.

Accordingly, the present invention has been made in view of the above-mentioned prior art, and has been made in consideration of an optical circuit having extremely low polarization dependence, particularly, a wavelength dependence in which the polarization dependence of the transmission blocking region and the PDL are extremely low. It is an object to provide a demultiplexing optical circuit.

[0011]

In order to solve the above-mentioned problems, an optical circuit according to the present invention comprises a core portion on a flat substrate, on which light made of quartz as a main material propagates, and a core portion of the core portion. An optical circuit composed of an optical waveguide having a cladding portion having a lower refractive index than the surrounding core portion, wherein the composition of the core portion and the cladding portion is adjusted in accordance with the planar substrate, whereby the optical waveguide is formed. 3x10 birefringence
^-5 or less, and a half-wave plate for compensating the birefringence is further provided.

[0012] Preferably, the cladding part is formed of at least boron (B),
Contains an element selected from the group consisting of germanium (Ge), phosphorus (P), nitrogen (N), and fluorine (F).

Preferably, the optical circuit is an arrayed waveguide type optical wavelength multiplexer / demultiplexer.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, boron (B), germanium (Ge), phosphorus (P), and nitrogen (N) are formed on a core portion and a cladding portion made mainly of quartz.
Add at least one or more dopants such as, depending on the substrate, manufactured by adjusting the composition, using a low birefringence optical waveguide, configure a wavelength multiplexing / demultiplexing optical circuit on the substrate,
The polarization dependence of the transmission blocking region is reduced in advance, and a half-wave plate is inserted into the optical circuit to compensate for the remaining polarization dependence, thereby achieving extremely low wavelength coupling of the PDL. A wave light circuit can be realized.

[0015] For this reason, the tolerance in the adjustment of the amount of dopant at the time of manufacturing the waveguide, the manufacturing accuracy of the half-wave plate, and the allowable range of the mounting accuracy at the time of inserting the wave plate can be widened. It is possible to improve the production yield of an extremely low wavelength multiplexing / demultiplexing optical circuit. As a result, the manufacturing cost is reduced and the field of optical communication and optical information processing, especially wavelength division multiplexing (W
DM) P which is mainly used in the field of communication systems
An optical wavelength multiplexing / demultiplexing optical circuit having an extremely low DL can be provided as a practical one at low cost and in large quantities. Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<Embodiment 1> FIG. 2 shows an arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit (Arrayed Waveguide) according to the present invention.
1 is a schematic perspective view illustrating an embodiment of (Grating: AWG).

In the drawing, reference numeral 2-1 denotes a Si substrate, 2-2.
And 2-3 are input / output waveguides, 2-4 and 2-5 are slab waveguides, 2-6 are arrayed waveguides, and 2-7 is 1
/ 2 wavelength plate. As shown in the figure, the AWG is composed of a plurality of waveguides having different lengths, and the input and output waveguides 2-1 and 2-2 and the two sector slab waveguides 2-4 and 2-4.
2-5 are integrated together on the substrate.

Next, an outline of an AWG manufacturing method will be described with reference to FIG. 3 (for details, see M. Kawachi, "Silica wa
veguides on silicon and their application to integ
rated-optic components ", Opt. and Quantum Electro
n., 22, pp. 319-416).

First, flame deposition (FHD) is performed on a silicon substrate (Si substrate) 3-1.
A quartz (SiO ₂ ) -based glass film is deposited by the method. That is, the lower clad glass layer 3-2 having a thickness of about 30 μm.
And a core glass layer 3-3 having a refractive index higher than that of the clad glass layer for transmitting light and having a thickness of about 6 μm. At this time, boron (B), germanium (Ge), phosphorus (P), and nitrogen (N) are used as dopants for quartz (SiO ₂ ), which is a main component, so that the waveguide birefringence becomes almost zero finally. Adjust (FIG. 3 (1)).

Next, reactive ion etching (RIE)
Is used to perform patterning into a desired optical waveguide shape. In the figure, the patterned core glass layer is shown in a convex cross section (FIG. 3 (2)).

Subsequently, the upper clad glass layer 3-4 having the same refractive index as that of the lower clad glass layer 3-2 is deposited again by using the FHD method, thereby producing an optical waveguide.
At this time, boron (B), germanium (Ge), phosphorus (P), and nitrogen (N) are used as dopants for quartz (SiO ₂ ), which is a main component, so that the waveguide birefringence becomes almost zero finally. adjust. Further, in this embodiment, the value of the relative refractive index difference Δ of the optical waveguide is adjusted to be 0.7% (FIG. 3 (3)).

In this embodiment, the glass layer is formed by the FHD method. However, the present invention is not limited to the FHD method, and other glass film synthesizing methods such as a CVD method and a sputtering method may be partially or entirely used. It is also possible to use. After preparing the optical waveguide, a dicing saw is used to move the optical waveguide length to about 2 equally in the arrayed waveguide section.
A groove having a width of 0 μm is formed, a polyimide wave plate is inserted, and the groove is fixed using an adhesive.

FIG. 4 is a diagram showing an example of the measurement result of the transmission spectrum of the AWG wavelength multiplexing / demultiplexing optical circuit manufactured based on the present embodiment at a wavelength of about 1.55 μm. It can be seen that an AWG wavelength multiplexing / demultiplexing optical circuit having a loss of about 4 dB and excellent transmission characteristics with an extinction ratio of 30 dB or more was manufactured.

FIG. 5 shows the result of measuring the PDL spectrum of this AWG wavelength multiplexing / demultiplexing optical circuit in the vicinity of a wavelength of 1.55 μm. In the PDL measurement, an external cavity wavelength-variable laser was used as a light source, and the measurement was performed by a PDL measuring device manufactured by JDS.

FIG. 6 shows the value of the PDL of the AWG with respect to loss. For reference, an example of the PDL value of the conventional AWG is also shown. It can be seen that the PDL value is kept low as compared with the conventional AWG. For example, comparing the PDL values at a loss of 10 dB, the conventional AWG has a PDL of 0.8 dB, whereas the AWG manufactured based on the present embodiment has
PDL to 0.3 dB (hereinafter referred to as A manufactured based on the present embodiment showing such a low PDL value).
WG is referred to as a very low PDL new AWG).

Next, in order to evaluate the production yield, a very low PDL new type AWG and a conventional type
Sample prepared, PD at wavelength with loss 10dB
L was evaluated. FIG. 7 shows the distribution of the PDL value.

The very low PDL new AWG generally has a PDL =
Good characteristics of 0.4 dB or less are obtained, and it can be seen that an extremely low PDL wavelength multiplexing / demultiplexing optical circuit can be manufactured with a higher yield than the conventional type.

[0028]

As described above, an optical circuit is constituted by an optical waveguide manufactured by adjusting the material composition of the core portion and the clad portion so as to have low birefringence, and using a half-wave plate. By compensating for the polarization dependence of the optical circuit, a wavelength multiplexing / demultiplexing optical circuit with extremely low polarization dependent loss (PDL) was realized. The production yield of the present optical circuit was better than that of the conventional optical circuit. Therefore, it is advantageous in terms of cost in producing and supplying a large number of wavelength multiplexing / demultiplexing optical circuits having a very low PDL.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a transmission spectrum of an AWG optical circuit and a monitoring light wavelength corresponding thereto.

FIG. 2 is a schematic perspective view showing an embodiment of an arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit according to the present invention.

FIG. 3 is a cross-sectional view for explaining a method of manufacturing an arrayed waveguide wavelength multiplexing / demultiplexing optical circuit according to the present invention;
(1) to (3) correspond to each step.

FIG. 4 is a graph showing transmission spectrum characteristics of an arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit according to the present invention in the vicinity of a wavelength of 1.55 μm.

FIG. 5 is a graph showing the loss and PDL wavelength characteristics near the wavelength of 1.55 μm of the arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit according to the present invention.

FIG. 6 shows the PDL of the arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit according to the present invention and the conventional arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit at a wavelength around 1.55 μm.
6 is a graph for comparing characteristics with respect to the loss of the semiconductor device.

FIG. 7 is a graph showing the distribution of PDL values of an arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit based on the present invention and a conventional arrayed waveguide type wavelength multiplexing / demultiplexing optical circuit at a loss of 10 dB near a wavelength of 1.55 μm. It is.

[Explanation of symbols]

 2-1 Si substrate 2-2 Input / output waveguide 2-3 Input / output waveguide 2-4 Slab waveguide 2-5 Slab waveguide 2-6 Array waveguide 2-7 1/2 wavelength plate

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-241304 (JP, A) JP-A-8-62441 (JP, A) JP-A 8-334637 (JP, A) JP-A 9-204 15434 (JP, A) JP-A-6-27342 (JP, A) JP-A-11-174246 (JP, A) JP-A-7-92326 (JP, A) Electronics Letters, Vol. 33 No. 13 (19th June 1997) pp. 1173-1174, S.M. Suzuki et. al. , IEEE Photonics Technology Letters, Vol. 6 No. 5 (May 1994) p.p. 626-628, Y. Inoue et. Al. , (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02B 6/12-6/14 G02B 6/28-6/293

Claims (3)

(57) [Claims] 1. A light constituted by an optical waveguide composed of a core portion formed of quartz as a main material through which light propagates, and a cladding portion having a lower refractive index than the core portion around the core portion. A circuit, wherein the size of the birefringence of the optical waveguide is adjusted to 3 × 10 ⁻⁵ or less by adjusting the composition of the core portion and the cladding portion corresponding to the planar substrate, and further, the birefringence is compensated. An optical circuit comprising a half-wave plate. 2. The cladding part is selected from the group consisting of at least boron (B), germanium (Ge), phosphorus (P), nitrogen (N), and fluorine (F) as an additional dopant of a quartz-based material. The optical circuit according to claim 1, further comprising an element to be used. 3. The optical circuit according to claim 1, wherein the optical circuit is an arrayed waveguide type optical wavelength multiplexer / demultiplexer.