JPH08201737A - Optical filter with variable passband width - Google Patents

Abstract

PURPOSE: To realize a band=pass width variable optical filter capable of varying a band-pass width without using a mechanical movable mechanism and separating one or plural channels from a wavelength multiplexed signal beam as necessary. CONSTITUTION: In an array waveguide type optical filter, core opening widths of respective waveguides 11a-11d of an inputting channel waveguide in a boundary between the inputting channel waveguide 11 and a first sector slab waveguide 12 are different from each other, and further, the core opening widths of respective waveguides of outputting channel waveguides 15a-15d in the boundary between a second sector slab waveguide 14 and the outputting channel waveguide 15 are different from each other, and waveguide type optical switches 21-26 are connected to the inputting channel waveguide 11 and the outputting channel waveguide 15 in cascade respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to optical wavelength division multiplexing (WDM).
The present invention relates to a bandpass optical filter used for separating wavelength-multiplexed signal light in a communication system and other purposes. In particular, the present invention relates to a variable passband optical filter having a filter characteristic of variable passband width.

[0002]

2. Description of the Related Art FIG. 6 shows a configuration of a Fabry-Perot resonator (hereinafter referred to as "FP resonator") used as a conventional optical filter. In the figure, the light of wavelength λ (optical frequency f = c / λ, where c is the speed of light) incident on the FP resonator 51 at an angle θ ′ repeats multiple reflection within the resonator, so that the output light intensity T is large. as the interference of light beams, T = (1-R) 2 / {(1-R) 2 + 4Rsin 2 (δ / 2)} ... is represented as (1). Here, R is the reflectance at the interface between the refractive indices n and n ′, and δ is δ = (4π / λ) nhcosθ = (4πf / c) nhcosθ (2) where h is the thickness of the resonator. Given. The angle θ is obtained from nsinθ = n′sinθ ′.

The intensity output characteristic of the FP resonator 51 is shown in FIG. 7 when plotted against the optical frequency f. The interval between resonance peaks is called FSR (Free Spectral Range), and is given by FSR = c / 2nhcosθ (3). The interval between the two optical frequencies at which the output light intensity is 1/2 the peak value is called the pass band width Δf _1/2, and Δf _1/2 = (c / 2πnhcosθ) (1-R) / √ R is given by (4).

Further, the ratio between the maximum value and the minimum value of the output light intensity is called an extinction ratio ER and is given by ER = (1-R) ² / (1 + R) ² (5). As an example, the thickness of the FP resonator is h = 82.
Assuming μm and refractive index n = 1.45, the relationship between the reflectance R, the pass band width Δf _1/2 and the extinction ratio ER is

[0005]

[Table 1]

[0006] From Table 1, the reflectance R of the FP resonator
It can be seen that the pass band width Δf _1/2 can be changed if the value can be changed.

[0007]

However, in the conventional FP resonator, since the reflectance R is controlled by coating the interface with a dielectric multilayer film, the reflectance R can be controlled by one FP resonator.
Was difficult to change. Further, as can be seen from Table 1, when the reflectance R is changed, there is a drawback that the extinction ratio ER is lowered.

Further, if the incident angle θ'of light is changed from the equation (4) to change the angle θ, the pass band width Δf _1/2 can be made variable. However, in this case, since the direction of the output light is changed and the optical axis is changed, there is a drawback that it cannot be used by arranging between fixed single mode fibers.
As described above, in the conventional optical filter using the FP resonator, the pass band width cannot be made variable in the single mode fiber system, so that a predetermined number of signal lights are arbitrarily extracted from the wavelength-multiplexed signal lights. I couldn't.

The present invention provides a variable-passband-width optical filter capable of varying the passband width without using a mechanically moving mechanism and separating one or a plurality of channels from the wavelength-multiplexed signal light as required. The purpose is to do.

[0010]

A variable pass band optical filter of the present invention is an arrayed waveguide type optical filter, wherein an input channel waveguide is provided at a boundary between the input channel waveguide and the first fan-shaped slab waveguide. , The core opening widths of the waveguides are different from each other, and the core opening widths of the waveguides of the output channel waveguides at the boundary between the second fan-shaped slab waveguide and the output channel waveguides are different from each other. A waveguide type optical switch is connected in cascade to the waveguide and the output channel waveguide, respectively.

[0011]

The variable pass band optical filter of the present invention turns on / off the waveguide type optical switch connected to the input channel waveguide and the waveguide type optical switch connected to the output channel waveguide, By selecting the input channel waveguide and the output channel waveguide for inputting / outputting the signal light, the pass band width can be varied according to the core opening width.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the configuration of an embodiment of a variable pass band optical filter of the present invention. In the figure, N (= 4) input channel waveguides 11 (11a,
11b, 11c, 11d), the first fan-shaped slab waveguide 1
2. A channel waveguide array 13 consisting of M waveguides that are sequentially lengthened by a predetermined waveguide length difference ΔL, a second fan-shaped slab waveguide 14, and N (= 4) output channel waveguides 15.
This is a configuration in which (15a, 15b, 15c, 15d) are sequentially connected. Further, in the preceding stage of the input channel waveguide 11, waveguide type optical switches 21, 22, which are 1 × 4 switches.
23 is connected to the output channel waveguide 15 and the
Waveguide type optical switch 24, 25, 2 which becomes a × 1 switch
6 is connected.

The waveguide length difference ΔL in the channel waveguide array 13 is set to a value given by ΔL = mλ / n _C (6) Here, m is the diffraction order (integer value), and n _C = β _C / k (β _C is the propagation constant of the channel waveguide, k = 2π / λ). One of the input waveguides 21a of the waveguide type optical switch 21 is used as a signal input waveguide, and one of the input waveguides 22a and 23 of the waveguide type optical switches 22 and 23 is used as its two output waveguides.
a is connected, and the input channel waveguides 11a to 11d are connected to the output waveguides of the waveguide type optical switches 22 and 23, respectively. In addition, the output channel waveguides 15a to 15
The input waveguides of the waveguide type optical switches 24 and 25 are connected to d, and the two input waveguides of the waveguide type optical switch 26 are connected to the output waveguides 24a and 25a of each one thereof, and the output of one of them is connected. The waveguide 26a is used as a signal output waveguide. Each of the waveguide type optical switches 21 to 26 constitutes a Mach-Zehnder interferometer and has thin film heaters T1 to T6 used for switching operation.

FIG. 2 is an enlarged view showing the structure in the vicinity of the first fan-shaped slab waveguide 12. FIG. 3 is an enlarged view showing the structure in the vicinity of the second fan-shaped slab waveguide 14. In the figure, r is the radius of curvature of the fan-shaped slab waveguides 12 and 14, and s
Is the waveguide spacing of the channel waveguide array 13, D _1a ~
D _1d and L _{1a to} L _1d are input channel waveguides 11 a to 11
d is the core opening width and taper length, D ₂ and L ₂ are the core opening width and taper length of the channel waveguide array 13, and D _{3a to} D
_3d, L _3a ~L _3d output channel waveguide 15a~15d
Of the core opening width and taper length. The core width of each of the input channel waveguide 11, the channel waveguide array 13, and the output channel waveguide 15 is 2a = 7 μm, and the core thickness is 2t =
7 μm, relative refractive index difference Δ = 0.75%.

Now, the input channel waveguide 1 shown in FIG.
1b, passes through the fan-shaped slab waveguide 12, enters the jth (j = 1 to M) th waveguide from the left of the channel waveguide array 13, and the waveguide and the fan-shaped slab waveguide 14 shown in FIG. Consider the light passing through to the output channel waveguide 15b. The distance OPL _j from the exit end A of the input channel waveguide 11b shown in FIG. 2 to the entrance end B of the j-th waveguide of the channel waveguide array 13 is OPL _j = r {(sin ψ _j −sin ξ _1b ). It is given by _{^{2 + (1-cosξ 1b -cosψ}} j) 2} 1/2 ≒ r (1-sinξ 1b · sinψ j) ... (7). Where ξ _1b is the input channel waveguide 1
1b (outgoing end A) is an angle of view with respect to the central axis, and ψ _j is an expected angle with respect to the central axis of the channel waveguide array 13 viewed from the j-th waveguide (incident end B).

Since the light passing through the jth waveguide of the channel waveguide array 13 is emitted from the jth waveguide from the right in FIG. 3, it is incident from the emission end C of the output channel waveguide 15b. distance OPR _j leading to the end D is given by _{OPR j = r {(sinψ j} + sinθ) 2 + (1-cosθ-cosψ j) 2} 1/2 ≒ r (1 + sinθ · sinψ j) ... (8) . Where ψ _j is the channel waveguide array 1
3 is the projected angle with respect to the central axis viewed from the j-th waveguide (output end C), and θ is the projected angle with respect to the central axis viewed from the output channel waveguide 15b (input end D). Note that the formula
In (7) and (8), ψ _j , ξ _1b , and θ << 1.

Since the channel waveguide array 13 is constructed so as to be sequentially elongated with a predetermined waveguide length difference ΔL,
The length of the j-th waveguide, when the first waveguide length and _{L 0 L 0 + (j-} 1) is given by [Delta] L. From the above, FIG.
The phase Θ _j of the light emitted from the input channel waveguide 11b, passing through the j-th waveguide of the channel waveguide array 13 and reaching the output channel waveguide 15b is Θ _j = β _S · OPL _j + Β _C {L ₀ + (j−1) ΔL} + β _S · OPR _j (9) Where β _S is the fan-shaped slab waveguide 12,
14 is a propagation constant.

Substituting equations (6), (7) and (8) into equation (9), Θ _j = β _S r {2 + sin ψ _j (sin θ-sin ξ _1b )} + β _C L ₀ + (j-1) 2 mπ … (10). Therefore, when θ = ξ _1b , Θ _j = 2β _S r + β _C L ₀ + (j−1) 2mπ (11). In the phase Θ _j of this light, the third term that is an integral multiple of 2π can be ignored, so that the phase Θ _j of the light that reaches the output channel waveguide 15b is the same as that in the channel waveguide array 13 when θ = ξ _1b . It can be seen that it is irrelevant to the incident position j. That is, when θ = ξ _1b , all the light emitted from the input channel waveguide 11b shown in FIG. 2 and passing through the channel waveguide array 13 can be collected in the output channel waveguide 15b shown in FIG. I understand.

Other output channel waveguides 15a to 15d
Similarly, for the input channel waveguides 11a to 11
position corresponding to d, that is, θ _3p = ξ _1p (p = a to d)
2 is arranged so that all the light emitted from the input channel waveguide 11p shown in FIG. 2 and passing through the channel waveguide array 13 is output channel waveguide 15 shown in FIG.
It will be focused on p.

FIG. 4 shows the structure of the waveguide type optical switch 21. (a) is a plan view, (b), (c) is an enlarged sectional view taken along the line AA 'and BB' of the plan view. The same applies to the other waveguide type optical switches 22 to 26. The waveguide type optical switch 21 has two waveguides 31 and 32 made of silica glass formed on the substrate 10 at two places with an interval of several μm.
Two directional couplers 33 and 34 are arranged close to m. The coupling rate of the directional couplers 33 and 34 is set to 50% by adjusting the waveguide interval and the length of the coupling section.
Two waveguides 3 between the two directional couplers 33 and 34
The lengths of 1 and 32 are set to be equal to each other, and the thin film heater T1 is installed above the cladding layer 35 of one waveguide (FIG. 4 (c)).

This optical switch element constitutes a Mach-Zehnder interferometer as a whole, and in the OFF state in which no current flows through the thin film heater T1, light input from the left end (21a) of the waveguide 31 (right end of the waveguide 32 ( The light output from the waveguide 23a) and input from the left end of the waveguide 32 is output from the right end (22a) of the waveguide 31 and enters a so-called cross port state. Further, when a current is passed through the thin film heater T1 to raise the temperature of the waveguide therebelow and a phase difference equivalent to π (radian) is generated between the two directional couplers by the thermo-optic effect, the waveguide 31 The light input from the left end (21a) is output from the right end (22a) of the waveguide 31, and the light input from the left end of the waveguide 32 is output from the right end (23a) of the waveguide 32 and is switched to a so-called through port state. .

Therefore, the signal light incident on the signal input waveguide (21a) in FIG. 1 is converted into the signal output waveguide (2a).
The propagation path reaching 4a) can be changed according to the driving states of the thin film heaters T1 to T6 of the six waveguide type optical switches 21 to 26. Table 2 shows the relationship between the driving states of the thin film heaters T1 to T6 and the propagation paths of signal light.

[0023]

[Table 2]

In the second fan-shaped slab waveguide 14 shown in FIG. 3, the change of the focused spot position x with respect to the light wavelength λ (or the optical frequency f) is Δx / when + x is taken in the right direction in FIG. Δλ = −rΔL / λ ₀ s, Δx / Δf = −rΔL / f ₀ s (12) Here, λ ₀ is the center wavelength of the wavelength division multiplexed signal, and f ₀ (= c / λ ₀ ) is the center optical frequency. Formula (12)
Since r, ΔL, s, λ ₀ , and f ₀ are all constant values,
It can be seen that the change of the focused spot position x with respect to the light wavelength λ (or the optical frequency f) is constant. Therefore, as shown in FIG. 3, the output channel waveguides 15a-1
It can be seen that when the core aperture width of 5d is different, the bandwidth of the signal light taken into each output channel waveguide is different depending on the core aperture width.

In the embodiment shown in FIGS. 1 to 3, the core opening widths D _{1a to} D _1d , D _{3a to} D _3d and the taper length L _1a of the taper portion are provided.
~L _1d, L _3a ~L _{3d is, D 1a = D 3a = 10μm} , L 1a = L 3a = 0.4mm D 1b = D 3b = 24μm, L 1b = L 3b = 2.5mm D 1c = D 3c = 40μm , L _1c = L _3c = 5.0 mm D _1d = D _3d = 54 μm, L _1d = L _3d = 7.0 mm. Other parameters are D ₂ = 23 μm,
L ₂ = 2 mm, r = 9.1 mm, s = 25 μm, ΔL = 63 μ
m, the diffraction order m = 59, and the number of waveguides in the channel waveguide array 13 was M = 120.

A mask was prepared on the basis of such parameters, and a variable pass band optical filter of this example was prepared using a silica optical waveguide. First, a SiO ₂ lower clad layer is deposited on a Si substrate by a flame deposition method, and then GeO ₂ is deposited.
After depositing a core layer of SiO ₂ glass to which was added as a dopant, transparent glass was formed in an electric furnace. Next, the core layer was etched using the pattern shown in FIG. 1 based on the above design to produce an optical waveguide portion. Finally, again SiO ₂
An upper clad layer was deposited, and a thin film heater and electric wiring were further deposited on a predetermined optical waveguide.

Regarding the demultiplexing characteristics of the variable passband optical filter manufactured in this way, the thin film heaters T1 to T6
FIG. 5 shows the measurement results obtained by setting four drive conditions shown in Table 2. The solid line is the driving state, the broken line is the driving state,
The dotted line corresponds to the driving state, and the alternate long and short dash line corresponds to the driving state.
As can be seen from this figure, the output channel waveguide 15
It was confirmed that the passband width (the entire frequency width at which the loss increases by 3 dB from the minimum value) becomes wider as the core opening width of is increased.

[0028]

As described above, the variable pass band width optical filter of the present invention includes one of the output channel waveguides (corresponding input channel waveguides) having different core aperture widths.
By selecting one, the pass band width according to the core opening width can be set. That is, by turning on / off the waveguide type optical switch and selecting one of the input channel waveguide and the output channel waveguide, an optical filter having a variable pass bandwidth can be obtained without using a mechanical movable mechanism. Can be realized.

Therefore, the variable pass band optical filter of the present invention can separate one or a plurality of channels from the signal light wavelength-multiplexed by the external control, so that it is suitable for large-capacity / long-distance optical communication using optical wavelength multiplexing. It can be used as an extremely useful element.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of a variable pass band width optical filter of the present invention.

FIG. 2 is an enlarged view showing a structure in the vicinity of a first fan-shaped slab waveguide 12.

FIG. 3 is an enlarged view showing a structure in the vicinity of a second fan-shaped slab waveguide 14.

FIG. 4 is a diagram showing a configuration of a waveguide type optical switch 21.

FIG. 5 is a diagram showing a result of measuring demultiplexing characteristics of a variable passband optical filter according to the present invention.

FIG. 6 is a diagram showing a configuration of an FP resonator used as a conventional optical filter.

FIG. 7 is a diagram showing intensity output characteristics with respect to an optical frequency f of the FP resonator.

[Explanation of symbols]

10 Substrate 11 Input Channel Waveguide 12 First Fan Slab Waveguide 13 Channel Waveguide Array 14 Second Fan Slab Waveguide 15 Output Channel Waveguide 21, 22, 23, 24, 25, 26 Waveguide Type Light Switch 31, 32 Waveguide 33, 34 Directional coupler 51 Fabry-Perot resonator (FP resonator)

Claims (1)

[Claims] 1. A channel waveguide array comprising an input channel waveguide, an output channel waveguide, and a plurality of waveguides that are sequentially lengthened by a predetermined waveguide length difference on a substrate, and the input channel. An array conductor in which a first fan-shaped slab waveguide that connects a waveguide and the channel waveguide array and a second fan-shaped slab waveguide that connects the channel waveguide array and the output channel waveguide are formed. In the waveguide type optical filter, the core opening widths of the waveguides of the input channel waveguide at the boundary between the input channel waveguide and the first fan-shaped slab waveguide are different from each other, and the second fan-shaped slab waveguide is provided. The core opening width of each waveguide of the output channel waveguide at the boundary between the output channel waveguide and the output channel waveguide is different from each other, and the waveguide type optical switch is cascaded to the input channel waveguide. And a waveguide type optical switch is connected in cascade to the output channel waveguide.