JPH07333446A - Optical multiplexer/demultiplexer - Google Patents

Abstract

PURPOSE:To provide an array waveguide diffraction type optical frequency multiplexer/demultiplexer which is improved in side lobe suppression ratio. CONSTITUTION:This waveguide diffraction type optical frequency multiplexer/ demultiplexer is formed by successively connecting an input waveguide 2, a first slab waveguide 3, plural channel waveguides 4 varying in length, a second slab waveguide 5 and an output waveguide 6 on a substrate 1. The multiplexer/ demultiplexer described above has a thermooptical phase shifter 11 for discretely controlling the phases of the plural channel waveguides 4 to discretely control the phases in such a manner that the errors of the phase change rates in the respective channel waveguides are kept constant. As a result, the errors of the phase change rates in the channel waveguides induced by a fluctuation in refractive index at the time of forming the waveguides are decreased and the side lobe suppression ratio is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an array waveguide diffraction grating type optical multiplexer / demultiplexer used for optical multiplex communication.

[0002]

2. Description of the Related Art In optical multiplex communication, an optical circuit having a high wavelength resolution is required to increase the transmission capacity. The array waveguide diffraction grating type optical multiplexer / demultiplexer is most useful as an optical circuit that realizes high wavelength resolution, and research and development are being actively conducted at present.

An example of using a conventional arrayed waveguide diffraction grating type optical multiplexer / demultiplexer as an optical frequency multiplexer / demultiplexer will be described below. FIG. 12 is a diagram showing the configuration of an arrayed waveguide diffraction grating type optical multiplexer / demultiplexer. An optical fiber on the transmission side is connected to the input waveguide 2 on the substrate 1, and optical frequency multiplexed light is incident on the input waveguide 2. The light spread by the diffraction effect in the input side slab waveguide 3 propagates through a plurality of channel waveguides forming the array waveguide diffraction grating 4, reaches the output side slab waveguide 5, and is further collected in the output waveguide 6. To be done. The arrayed-waveguide diffraction grating 4 is designed so that the lengths of the channel waveguides are different, and the light extracted from the output waveguide 6 is only the light having the same phase out of the light propagated through the respective channel waveguides. , This circuit will have demultiplexing characteristics.

The array waveguide diffraction grating 4 is characterized in that its frequency resolution is proportional to the optical path length difference (ΔL) between the channel waveguides forming the array waveguide diffraction grating 4 and the adjacent waveguides. That is, by designing ΔL large, it is possible to combine and demultiplex multiplexed light having a narrow optical frequency interval. The solid line of the graph shown in FIG. 13 shows the transmission frequency characteristic of the conventional arrayed waveguide diffraction grating type optical frequency multiplexer / demultiplexer. Using this optical multiplexer / demultiplexer, it is possible to demultiplex the light of the multiplexing number of 11 at the optical frequency interval of 10 GHz.

Generally, the most important criterion when evaluating an optical frequency multiplexer / demultiplexer is the sidelobe suppression ratio defined by the ratio of the maximum transmittance to the maximum transmittance in the sidelobe. It can be said that the larger the side lobe suppression ratio, the better the characteristics of the multiplexer / demultiplexer. The side lobe suppression ratio of the above-mentioned conventional arrayed waveguide diffraction grating type optical frequency multiplexer / demultiplexer is 10
It was dB. The broken line in FIG. 13 shows the transmitted light frequency characteristic expected from theory. The theoretically expected sidelobe suppression ratio is 30 dB or more, and there is a considerable difference in comparison with the actually created multiplexer / demultiplexer. The reason for this is considered to be that the optical path length difference ΔL is as large as mm, and an error occurs in the amount of phase change in the channel waveguide. Actually, if the refractive index n fluctuates by about 10 −4,
An error of 0 degree phase change is easily induced. Therefore, in the array waveguide diffraction grating type optical frequency multiplexer / demultiplexer, in order to improve the side lobe suppression ratio, it is necessary to reduce the error in the amount of phase change in each channel waveguide by some method.

An example in which an optical phase controller is inserted in a plurality of channel waveguides of an array waveguide diffraction grating type optical frequency multiplexer / demultiplexer is disclosed in Japanese Patent Laid-Open No. 5-323246.
14 and 15 are diagrams showing the configuration of the arrayed waveguide diffraction grating type optical frequency multiplexer / demultiplexer disclosed in the above publication. 12 that are the same as those in FIG. 12 are the same components. In the multiplexer / demultiplexer of FIG. 14, an optical frequency selection function is provided by inserting thermo-optical phase shifters 7 having different lengths corresponding to the lengths of a plurality of channel waveguides. In the demultiplexer, the transmission wavelength width variable function is provided by inserting the thermo-optical phase shifter 8 in every other plurality of channel waveguides.

[0007]

However, as shown in FIG.
Further, in the configuration in which the thermo-optic phase shifters 7 and 8 are connected in cascade like the multiplexer / demultiplexer shown in FIG. 15, the phase of each channel waveguide cannot be controlled individually, so that it is not possible to improve the side lobe suppression ratio. Can not.

In view of the above problems, it is an object of the present invention to provide an array waveguide diffraction grating type optical frequency multiplexer / demultiplexer capable of improving the sidelobe suppression ratio.

[0009]

In order to solve the above-mentioned problems, according to claim 1 of the present invention, an input waveguide, a first
Slab waveguide, multiple channel waveguides with different lengths,
In the array waveguide diffraction grating type optical multiplexer / demultiplexer in which the second slab waveguide and the output waveguide are sequentially connected on the substrate, and the optical phase controller is installed in the plurality of channel waveguides, the optical phase control is performed. The instrument has means for individually phase controlling each channel waveguide. In the second aspect, the waveguide is a quartz optical waveguide, and the phase control means is a thermo-optical phase shifter including a thin film heater. Further, in the present invention, the waveguide is a silica optical waveguide, and the phase control means irreversibly changes the stress acting on the core portion of the channel waveguide by trimming to adjust the refractive index of the waveguide. The obtained stress imparting film was used.

[0010]

In the optical multiplexer / demultiplexer of the present invention, by providing means for individually controlling the phase of a plurality of channel waveguides, the phase can be controlled so that the error in the amount of phase change in each channel waveguide becomes constant. It can be controlled individually. Therefore, it is possible to reduce the error in the amount of phase change in the channel waveguide that is induced by the fluctuation of the refractive index when creating the waveguide. Therefore, the array waveguide diffraction grating type optical frequency multiplexing / demultiplexing with improved sidelobe suppression ratio can be achieved. Can be realized.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a plan view of an array waveguide diffraction grating type optical frequency multiplexer / demultiplexer according to a first embodiment of the present invention.
On the substrate 1, an input waveguide 2 (for example, five), a first slab waveguide 3, and a plurality of channel waveguides 4 having different lengths are provided.
(For example, 21), the second slab waveguide 5, and the output waveguide 6 (for example, 5) are sequentially connected and provided. Further, the channel waveguide 4 has an optical phase controller 1
0 is loaded. The light phase controller 10 includes a thin film heater 11 as a thermo-optical phase shifter and the thin film heater 1.
1, which is composed of an electric wire 12 for supplying a current, an independent electrode 13 and a common electrode 14, supplies electric power to the electrodes 13 and 14 and changes the temperature of the thin film heater 11 to change the refractive index of the waveguide. Is. The electric wiring 12 from the thin film heater 11 is connected to the independent electrode 13 without intersecting with the electric wiring 12 from the other thin film heaters 11, so that the phase of each waveguide of the channel waveguide 4 can be individually controlled. It has become.

In making the optical multiplexer / demultiplexer, an underclad film of silica glass is first deposited to 30 μm on the silicon substrate 1 as shown in FIG. 2 by using a flame deposition method,
Next, silica glass with Ge added was deposited to 7 μm as a core film. The refractive index of the core film was 0.75% higher than that of the silica glass of the underclad and the overclad. Next, unnecessary portions of the core film were removed by photolithography and reactive ion etching to form the waveguide shape shown in FIG. Further, a buried type three-dimensional optical waveguide 4 was produced by depositing an overclad film of quartz glass by a flame deposition method. The waveguide core size was 7 μm × 7 μm. Finally, the optical phase controller 10 was manufactured by photolithography and metal deposition.

In designing the optical waveguide, the pitch of the arrayed waveguide diffraction grating (the distance between the slab waveguides 3 and 5 of the channel waveguides 4 constituting the arrayed waveguide) d is set to 2
5 μm, the radius of curvature f of the slab waveguide is 3.312 m
m, and the optical path length difference ΔL between the channel waveguides forming the arrayed waveguide and the adjacent waveguides is 2881 μm. At this time,
The line dispersion at the end of the slab waveguide 5 of the output waveguide 6 becomes 25 μm intervals per 10 GHz in the wavelength 1.55 μm band, so that the interval of the input portion of the output waveguide 6 is 25 μm intervals so that the wavelength multiplexing interval of 10 GHz can be obtained. did.
Further, the number of input waveguides is 5, the number of channel waveguides is 21, and the number of output waveguides is 5.

FIG. 3 shows the transmitted light frequency characteristic of the fabricated arrayed waveguide diffraction grating type optical frequency multiplexer / demultiplexer, and the side lobe suppression ratio is compared with a theoretically expected value of 30 dB or more. It's getting worse.

In this embodiment, the amount of phase change in each waveguide of the multiplexer / demultiplexer was measured using the measurement system as shown in FIG. In FIG. 4, A is an array waveguide diffraction grating type optical multiplexer / demultiplexer as an optical circuit, 20 is a light emitting diode with a wavelength of 1.55 μm, 21 is a distributed feedback laser with a wavelength of 1.3 μm, and 22a and 22b are optical. Fiber 3dB coupler, 2
3a, 23b and 23c are objective lenses, 24 is a prism,
Reference numeral 25 is a reflector, 26 is a movable stage, 27 is one port of the coupler 22b, and 28 is a wavelength of 1.3 μm.
Is a dichroic mirror that reflects light and transmits a wavelength of 1.55 μm, 29a and 29b are photodetectors, 30 is a fringe counter, 31 is a waveform recorder, and 32 is a computer. In FIG. 4, light emitted from a light emitting diode having a wavelength of 1.55 μm and a distributed feedback laser having a wavelength of 1.3 μm is
It is divided into two by the coupler 22a. One of the two divided lights enters the coupler 22b via the optical circuit A. The other half of the divided light becomes reference light, which is collimated into a parallel beam by the objective lens 23b, reflected by the prism 24, reflected by the reflector 25 in parallel, and again passes through the prism 24 and the objective lens 23b. It is condensed by and is incident on the coupler 22b. The light that has passed through the optical circuit A and the reference light are combined by a coupler and extracted from the port 27 of the coupler 22b. The light emitted from the port 27 of the coupler 22b is collimated into a parallel beam by the objective lens 23c, the light of wavelength 1.55 μm passes through the dichroic mirror 28, and the light of wavelength 1.3 μm is reflected by the photodetector 29a. It is guided to the photodetector 29b. Here, the stage 26 is used to move the reflector 25 in parallel (in the direction of the arrow) to the incoming and outgoing beams to change the optical path length of the reference optical path. The photodetector 29b detects a beat signal that changes by a half cycle when the optical path length of the reference optical path changes by λ / 2. The fringe counter 30 detects the change in the beat signal and generates a clock pulse every time the optical path length of the reference optical path changes by λ / 2. On the other hand, the photodetector 29a detects an interferogram generated by the interference between the light wave propagating through the optical circuit A and the reference light. The waveform recorder 31 set to the external clock mode operates the fringe counter 3
The signal generated by the photodetector 29a (interferogram generated by the interference between the light wave propagating through the optical circuit and the reference light) is sampled with the clock generated by 0.

The light waves propagating through the respective paths in the optical circuit A interfere with the reference light only when the optical path lengths of the respective optical paths are equal to each other within the coherence length of the light source. Since the coherence length of the light source is sufficiently shorter than the optical path length difference between adjacent channel waveguides, the beat signal Vk (x) between the light propagating through each channel waveguide and the reference light is the other channel waveguide in the optical circuit. The beat signal Vj (x) (j ≠ K) of the light wave propagating through and the reference light does not overlap. That is, the interferogram measured by changing the optical path length of the reference optical path is calculated from 21 isolated beat signals Vk (x) (k = 1, 2, ..., 21) corresponding to each path in the optical circuit. It is formed.

Therefore, the interferogram is taken into the computer 32 from the waveform recorder 31, the beat signal Vk (x) including the information of each path is separated, and the coordinate xk near the peak value of the separated beat signal Vk (x) is obtained. Select and introduce a coordinate yk with xk as the origin, and beat signal Wk (yk) (= Vk
The fast Fourier transform of (x)) for yk was calculated. The amplitude bk (σ) obtained by the Fourier transform and the phase change amount φk (σ) received by the light propagating through each channel waveguide are connected by the following relational expression.

G (σ) ak (σ) exp (−iφk (σ)) = exp (−2πiσxk) · ∫Wk (yk) · exp (−2πiσyk) dyk (1) = exp (−2πiσxk) · bk ( σ) · exp (-iψk (σ)) (2) where Wk (yk) = Vk (yk + xk). Therefore, the amplitude ak (σ) of the light propagating through each path is (3)
From the equation, the phase change amount φ that the light propagating through each path receives
k (σ) is inclusive of 2πm = (m = 1,2,3…)
It can be determined from equation (4).

Ak (σ) = bk (σ) / g (σ) (3) φk (σ) = 2πσxk + ψk (σ) + 2πmk (4) = Φk (σ) + 2πmk (5) Φk (σ) = 2πσxk + ψk (Σ) (6) The amount of phase change obtained by calculating the Fourier transform of the beat signal in this way is Φk (σ) in Eq. (6) and is expressed by φk (σ) in Eq. (5). There is a difference of 2πmk from the actual value. However, the characteristic of the optical circuit is that the phase difference between arbitrary paths Φk (σ) −φj (σ) = (Φk (σ) −Φj (σ)) + 2π (mk−mj) (7) is from the design value. Because it is decided by the deviation,
When the second term of equation (7) is sufficiently smaller than the first term, that is, when the optical path length difference between the paths is sufficiently longer than the wavelength of the light source, the second term is ignored and the first term is ignored. Can be the phase difference between arbitrary paths. Since this example satisfies this condition, the amount of phase change received by the light propagating through each path was derived from equation (6).

FIG. 5 shows the error distribution of the phase change amount in the channel waveguide measured by the above process. From this figure, it is understood that in the produced optical multiplexer / demultiplexer, the phase error is distributed over a wide range of ± 180 degrees, which causes the deterioration of the sidelobe suppression ratio.

FIG. 6 shows changes in the phase error distribution when the power of only the thermo-optic phase shifter loaded on the center (11th) waveguide of the 21 channel waveguides is changed. is there. From this figure, it can be seen that only the phase of the 11th waveguide that supplies power to the thermo-optic phase shifter changes, and there is almost no effect on the adjacent waveguides. This is because the distance between the adjacent channel waveguides is set to be sufficiently large as 1.5 mm. Further, the phase of the waveguide is 0.48 (degrees / m) in proportion to the power supplied to the thermo-optic phase shifter.
It can be seen that it can be changed at the rate of w).

Based on the above data, the thermo-optic phase shifters loaded in the respective waveguides were driven so that the phase error of the 21 channel waveguides would be constant. FIG. 7 shows the resulting phase error distribution of the optical frequency multiplexer / demultiplexer, and FIG.
Shows the transmitted light frequency characteristic. The wavy line in FIG. 8 shows the transmitted light frequency characteristics before driving the phase shifter shown in FIG. By independently driving the thermo-optic phase shifter loaded in the channel waveguide and suppressing the phase error distribution within ± 5 degrees as shown in FIG. 7, the side lobe suppression ratio is improved and the transmission loss in the transmission band is increased. It was possible to realize the reduction of

FIG. 9 is a plan view of an array waveguide diffraction grating type optical frequency multiplexer / demultiplexer according to a second embodiment of the present invention. The structure is almost the same as that of the first embodiment,
The difference is that a stress applying film 15 for changing the stress exerted on the channel waveguide portion is loaded instead of the thermo-optical phase shifter.

The wavy line in FIG. 10 shows the transmitted light frequency characteristics of the fabricated array waveguide diffraction grating type optical frequency multiplexer / demultiplexer, and the wavy line in FIG. 11 shows the error distribution in the phase change region in the channel waveguide. Is. In the produced optical multiplexer / demultiplexer, the phase error is distributed in a wide range of ± 180 degrees, which causes the deterioration of the side lobe suppression ratio.

In this embodiment, the stress imparting film 15 made of amorphous silicon is formed, and a part of the stress imparting film 15 is heated by an Ar ion laser beam to be polycrystallized. The stress was partially relieved and the amount of phase change in each waveguide was controlled. Generally, the stress-applying film changes the stress in both the horizontal and vertical directions with respect to the substrate surface, and the amount of stress change in each direction changes as the width of the stress-applying film to be loaded changes. Therefore, in this embodiment, the width of the stress applying film is set to 95 μm so that the stress applied to the channel waveguide is the same in both directions. By doing so, the amount of change in the refractive index in both directions due to stress relaxation is made the same.

In this embodiment, an Ar ion laser was used to polycrystallize the amorphous silicon film, but He--C was used.
An excimer laser such as a d laser, a XeCl or KrF laser, a Ti sapphire laser, a YAG laser, a solid-state pulse laser such as an alexandrite laser, or a CO2 laser can also be used. The amount of amorphization of the stress-applying film loaded on each channel waveguide was determined for each waveguide so that the phase error would be constant, based on the data in FIG.

The solid lines in FIGS. 10 and 11 are diagrams showing the transmitted light frequency characteristics and the error distribution characteristics of the amount of phase change obtained as a result of partially polycrystallizing the stress-applying film according to the above steps. By suppressing the phase error distribution within ± 5 degrees, it was possible to improve the sidelobe suppression ratio and reduce the transmission loss in the transmission band.

In the first embodiment, since the electric power is constantly supplied to the thin film heater, it is necessary to control the temperature of the substrate, but in the present embodiment, the phase change amount in the waveguide is not applied to each channel. Since it is reversibly changed, it is very effective in that it is not necessary and can be applied to an optical multiplexer / demultiplexer more easily.

[0030]

As described above, according to the first and second aspects of the present invention, the side lobe suppression ratio can be greatly improved as compared with the conventional one. As a result, the range of application of the arrayed waveguide grating type optical frequency multiplexer / demultiplexer in optical communication can be expanded. Further, according to claim 3, in addition to the above effects, there is an advantage that it can be applied more simply.

[Brief description of drawings]

FIG. 1 is a plan view showing the configuration of a first embodiment.

FIG. 2 is an enlarged cross-sectional view taken along the line AA ′ in FIG.

FIG. 3 is a diagram showing transmitted light frequency characteristics of an optical frequency multiplexer / demultiplexer before driving the thermo-optic phase shifter.

FIG. 4 is a diagram showing an example of a measurement system for deriving a phase change amount.

FIG. 5 is a diagram showing an error distribution of the amount of phase change in the channel waveguide before driving the thermo-optic phase shifter.

FIG. 6 is a diagram showing a phase error distribution when the power of only the thermo-optic phase shifter of the central waveguide is changed.

FIG. 7 is a diagram showing an error distribution of the amount of phase change in the channel waveguide after driving the thermo-optic phase shifter.

FIG. 8 is a diagram showing transmitted light frequency characteristics of the optical frequency multiplexer / demultiplexer after driving the thermo-optical phase shifter.

FIG. 9 is a diagram showing a configuration of a second embodiment.

FIG. 10 is a diagram showing transmitted light frequency characteristics of the optical frequency multiplexer / demultiplexer before and after controlling the amount of phase change.

FIG. 11 is a diagram showing an error distribution of the amount of phase change in the channel waveguide before and after controlling the amount of phase change.

FIG. 12 is a diagram showing a configuration of a conventional arrayed waveguide grating type optical frequency multiplexer / demultiplexer.

FIG. 13 is a diagram showing a transmitted light frequency characteristic of a conventional array waveguide diffraction grating type optical frequency multiplexer / demultiplexer.

FIG. 14 is a diagram showing a configuration of a conventional array waveguide diffraction grating type optical frequency multiplexer / demultiplexer in which an optical phase controller is inserted in a plurality of channel waveguides.

FIG. 15 is a diagram showing a configuration of a conventional array waveguide diffraction grating type optical frequency multiplexer / demultiplexer in which an optical phase controller is inserted in a plurality of channel waveguides.

[Explanation of symbols]

1 ... Substrate, 2 ... Input waveguide, 3 ... Input side slab waveguide,
4 ... Array waveguide diffraction grating, 5 ... Output side slab waveguide,
6 ... Output waveguide, 10 ... Optical phase controller, 11 ... Thermo-optical phase shifter (thin film heater), 12 ... Electrical wiring, 13, 14
... electrode, 15 ... stress imparting film, A ... optical circuit (array waveguide diffraction grating type optical multiplexer / demultiplexer), 20 ... light emitting diode with wavelength 1.55 .mu.m, 21 ... distributed feedback laser with wavelength 1.3 .mu.m, 22a, 22b ... Optical fiber 3 dB coupler, 23
a, 23b, 23c ... Objective lens, 24 ... Prism, 2
5 ... Reflector, 26 ... Stage, 27 ... One port of coupler, 28 ... Dichroic mirror with 1.3 .mu.m reflection and 1.55 .mu.m transmission, 29a, 29b ... Photodetector, 3
0 ... Fringe counter, 31 ... Waveform recorder, 32 ... Computer.

Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location G02B 6/28 D (72) Inventor Masayuki Okuno 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Within the corporation

Claims (3)

[Claims] 1. An input waveguide, a first slab waveguide, a plurality of channel waveguides having different lengths, a second slab waveguide,
An array waveguide diffraction grating type optical multiplexer / demultiplexer in which the output waveguides are sequentially connected on the substrate, and the optical phase controllers are installed in the plurality of channel waveguides. An optical multiplexer / demultiplexer having means capable of individually controlling the phase of each. 2. The optical multiplexer / demultiplexer according to claim 1, wherein the waveguide is a silica-based optical waveguide, and the phase control means is a thermo-optical phase shifter composed of a thin film heater individually inserted in each waveguide. Wave instrument. 3. The waveguide is a silica optical waveguide, and the phase control means can irreversibly change the stress acting on the core portion of the channel waveguide by trimming to adjust the refractive index of the waveguide. The optical multiplexer / demultiplexer according to claim 1, wherein the optical multiplexer / demultiplexer is a stress imparting film.