JPH01306802A - Optical frequency filter - Google Patents

Abstract

PURPOSE:To obtain a large RSR value without shortening the optical path length of a Fabri-Perot resonator by incorporating an asymmetrical Mach- Zehnder optical intereferometer circuit in the Fabri-Perot resonator. CONSTITUTION:The asymmetrical mach-Zehnder optical interferometer circuit 16 is incorporated in the Fabri-Perot optical interferometer circuit and signal light passes through the Mach-Zehnder optical interferometer circuit 16 several times reciprocally. Therefore, the asymmetrical Mach-Zehnder optical interferometer circuit 16 which has a waveguide length difference 2l prescribed by C/2n.l (C: light velocity, n: refractive index of optical waveguide) is provided in the optical waveguide which constitutes the Fabri-Perot resonator 15 and the length L of the optical waveguide a the Fabri-Perot resonator 15 can be set larger than the l value. Consequently, the optical frequency filter is constituted which has a (Free Spectrum Range) FSR value without shortening the optical path length of the Fabri-Perot resonator 15.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical frequency filter used in the field of optical communications, particularly in the field of optical frequency multiplexing communications.

[Prior art] Low loss wavelength range of 0 and 3 for silica-based optical fiber transmission line
An optical frequency multiplexing transmission method that densely multiplexes and transmits multi-channel optical signals at an optical frequency interval of approximately 5 GHz (approximately 0.4 people when converted to an optical wavelength interval) in the μm band or 1.55 μm band. , is attracting attention as a future ultra-large capacity transmission method.

An optical frequency filter can be cited as one of the optical components required to realize such an optical frequency multiplexing transmission system. Optical frequency filters are important optical components that selectively extract desired channels from multiplexed optical signals.

A Fabry-Bérot optical resonator has been known as an optical frequency filter. Figure 4 (A) and ([1
) are a plan view and an explanatory diagram of optical frequency response characteristics, respectively, of a conventional Fabry-Bérot optical resonator configured on a flat substrate. In FIG. 4 (8), 1 is a planar substrate, 2 is a single mode optical waveguide formed on the planar substrate 1,
3a and 3b are the input end and output end of the optical waveguide 2, respectively, and 4a and 4b are reflective mirrors arranged on each end face of the input end 3a and the output end 3b. An example of specific materials for each of these parts is as follows.

Substrate 1: Optical crown glass plate Single mode optical waveguide 2: Single mode optical waveguide formed on substrate 1 by cesium ion diffusion method Reflectors 4a, 4b: Optically polished input/output ends 3a, 3b deposited by vapor deposition Multilayer interference film (reflectance of about 90-99).

Figure 4 CB) shows the output end 3 b h) when the optical frequency of the signal light (for example, 1.55 μm wavelength band) of the intensity Pin incident on the input end 3 a is changed in Figure 4 (8).
The emitted light intensity P. This shows the change in ut. In this optical frequency response, equally spaced peaks are seen along the optical waveguide 2, corresponding to the resonance phenomenon of the signal light traveling back and forth between the reflecting mirrors 4a and 4b. In this way, Figure 4 (^)
The Fabry-Bello resonator operates as an optical frequency filter that passes optical signals matching the peak optical frequency.

The peak interval in Fig. 4 CB) is FSR (Free
Spectrum Range) is an important parameter called Spectrum Range, and it is well known that it has the following relationship with the single mode optical waveguide length l.

(C is the speed of light, n is the refractive index of the optical waveguide) For example, in order to achieve FSR-100G)+2 with the above-mentioned ion diffusion optical waveguide (n41.54), A#l
It is necessary to set it to nm.

Furthermore, in practice, the optical frequency filter is required to be able to tune one channel out of a plurality of channels by changing its selected optical frequency (peak optical frequency) as desired. It is known that in order to perform such a tuning effect in the Fabry-Bello resonator shown in Fig. 4, it is sufficient to slightly change the optical path length 2n-It of the resonator and move the resonance peak optical frequency position. There is. 2n
- When J2 is changed by an amount equivalent to one wavelength, the peak optical frequency moves by an amount equivalent to the FSR value. Therefore, for example, if the No. 3 light is multiplexed at intervals of Δf-5Gllz, the Fabry-Bello resonator can multiplex up to the maximum FSn/Δf channels, that is, FSR-100GI (in the case of z, up to about 20 channels). In this case, when dealing with multiplexed signal lights of 20 channels or more, two or more channels of signal lights separated by an interval of FSr1 can be simultaneously tuned by Fabry tuning.
There is a possibility that the light will pass through the bellows resonator type optical frequency filter, which is undesirable.

[Problems to be Solved by the Invention] In order to finely adjust the optical path length 2n-1, an electro-optic effect, a thermo-optic effect, etc. can be used depending on the material of the single mode optical waveguide 2. For example, in the example of the ion-diffusing glass single-mode optical waveguide described above, thermo-optic effects can be utilized. The thermo-optic coefficient of the glass material is dn/dT 410-
'/ Since the temperature change ΔT required to obtain an optical path length change equivalent to 1 wavelength λ = 1.55 μm is ΔT
-dn/dT' 2J2-1.55 a m, 14
In the case of 1 mm, the temperature is as high as ΔT-80° C., which requires a special heat dissipation design, which has been a major obstacle in constructing a practical tunable optical frequency filter. If you try to further increase the FSR value to widen the tunable range, the required Δτ value will further increase, so in practice, a Fabry-Bello resonator type optical frequency filter with an FSR value of about 50 GHz or more is used as a low-loss filter. Even if an attempt was made to construct a tuner using a glass optical waveguide system, this would be difficult and a major obstacle in providing a tuner for optical frequency multiplexing communications.

Even if an optical frequency filter is configured using a LiNbO3 optical waveguide that can utilize the electro-optic effect, the larger the FSR value, the smaller the λ value, and the smaller the overall length of the electro-optic phase shifter. Unavoidably, there is a problem in that the applied voltage required for tuning due to the change in optical path length becomes relatively high.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks and provide an optical frequency filter with a wide optical frequency tuning range.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides a flat substrate and a unit disposed on the flat substrate and provided with reflecting mirrors at both ends to constitute an optical resonator. It is characterized by comprising a one-mode optical waveguide and an asymmetric Mach-Zehnder optical interferometer circuit inserted in the middle of the optical waveguide on a plane substrate and configured together with a part of the optical waveguide and a second optical waveguide. .

Here, the reflecting mirror can be a waveguide-type optical reflection circuit composed of a loop-shaped optical waveguide and a directional coupler.

Furthermore, a phase shifter that finely adjusts the optical path length and adjusts the filter selection optical frequency can be provided on the optical waveguide that constitutes the Mach-Zehnder optical interferometer and on the optical waveguide that constitutes the optical resonator.

All optical waveguides are silica-based single mode optical waveguides constructed by embedding a silica-based glass core in a silica-based glass cladding layer placed on a flat substrate, and the phase shifter is located in the core. It can consist of a thin film heater formed on top of an upper cladding layer.

[Function] In the present invention, an asymmetric Mach-Zehnder optical interferometer circuit is incorporated into a Fabry-Bello optical interferometer circuit,
By configuring the Mach-Zehnder type optical interferometer circuit so that the signal light passes back and forth many times, the length defined by equation (1) above is created in the middle of the optical waveguide that constitutes the Fabry-Bello resonator. An asymmetric Mach-Zehnder optical interferometer circuit with a waveguide length difference of 2Jl is provided, and the optical waveguide length as a Fabry-Bello resonator can be set larger than the λ value.

Thereby, in the present invention, a large FSR value can be achieved without shortening the optical path length of the Fabry-Bello resonator.

It is possible to construct an optical frequency filter having the following. Therefore, sufficient space is secured on the optical path to install a phase shifter, such as a thin film heater phase shifter, to turn this optical frequency filter into a tuner pull, which places an excessive burden on the phase shifter. .

It is possible to tune to the desired optical frequency value without any interference.

In contrast to conventional optical frequency filters, in the present invention, the optical waveguide length for which the optical path length should be finely adjusted can be set to a sufficiently long length without being slightly constrained. In the case of changing the wavelength by tuning, the target optical waveguide length is sufficiently longer than the λ value, so there is an advantage that the temperature increase width ΔT- can be small.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Embodiment 1 FIGS. 1(8) and 1(B) are a plan view and an enlarged cross-sectional view taken along line AA', respectively, showing a first embodiment of the present invention.

In FIGS. 1(A) and (B), a long silica-based single mode waveguide 12 is arranged on a silicon substrate ll as a flat substrate, and both end faces of this silica-based single mode waveguide 12 are arranged. Reflecting mirrors 14a and 1 are provided on 13a and 13b, respectively.
4b to constitute the Fabry-Perot resonator 15.

The biggest difference from the conventional configuration shown in Figure 4 (8) is
The point is that an asymmetric Mach-Zehnder optical interferometer circuit 16 is inserted in the middle of the optical waveguide 12. That is, the asymmetric Mach-Zehnder optical interferometer circuit 16 connects two directional couplers 17a and 17b with an optical interferometer arm waveguide 18a, which is also a part of the optical waveguide 12, and another arm waveguide 18b. It consists of: here,
The difference in length Δρ2□ between arm waveguides 18a and 18b is (
Corresponding to 2A in equation 1), it is set to satisfy the following equation in accordance with the desired FSR value.

Single mote optical waveguides 12 and 18b in this embodiment
is 5iCj24. on the silicon substrate 11. This is a silica-based single-mode optical waveguide fabricated by a known combination of glass film deposition technology using flame hydrolysis reaction of glass-forming raw material gas such as Tic Bun 4 and reactive ion etching technology (Reference: Kawachi et al. Masao "Microfabrication of silica-based optical waveguides", Journal of the Micro-Optics Research Group of the Optics Conference of the Japan Society of Applied Physics 198B.

41vo1. No, 2. pp. 33-38). The silica-based single mode optical waveguide is as shown in Figure 1 (CB).
It is embedded in a cladding layer 19 with a thickness of about 50 μm, and the core part dimensions of the optical waveguide are about 7 μm x 7 μm.
The relative refractive index difference between the claddings was approximately 0.7 tones, and the coupling ratio of the directional couplers 17a and 17b was designed to be 5096.

On the optical waveguides 12 and 18b, thin film heater phase shifters 20a, 20b, and 21 are formed as phase shifters for finely adjusting the optical path length using the thermo-optic effect. This thin film heater phase shifter can be formed, for example, by depositing a chromium metal film using a vacuum deposition technique.

Figure 2 (A> and CB) is similar to Figure 1 (A) and (B).
) is a frequency characteristic diagram illustrating the operation of the optical frequency filter according to the embodiment of the present invention shown in FIG. First, FIG. 2(A) shows the characteristics in FIG. 1(A) assuming that there are no reflecting mirrors 14a and 14b, and the optical frequency of the signal light incident on the input end face 13a is changed. 3 shows the optical frequency characteristics of the intensity of light emitted from the output end face 13b at the time of FIG. That is, FIG. 2(A) shows the optical frequency characteristic of the Mach-Zehnder optical interferometer circuit 5 as a single unit, and as is well known, it has a sinusoidal response characteristic, and its peak optical frequency f.

The period of is expressed by equation (2). Here, AJZvz is the difference in length between the two arm optical waveguides 18a and 18b constituting the Mach-Zehnder optical interferometer circuit 5, so if the difference is maintained at ΔJ2MZ, the arm optical waveguides +8a and 1
Each absolute length of 8b can be set freely.

In order to change the peak optical frequency f by the FSR value, the optical path length of the arm optical waveguide 18b can be changed by an amount equivalent to one wavelength by increasing the temperature of the core portion using the thin film heater phase shifter 21. Because the heater length can be set to be long without being constrained by the ΔρMZ value due to the above-mentioned circumstances, the necessary temperature increase width ΔT can be significantly reduced.

However, since the optical frequency characteristic shown in FIG. 2 (8) described above has poor frequency selectivity as an optical frequency filter, the present invention uses a Fabry filter in which reflecting mirrors 14a and 14b are provided on input and output end faces 13a and 13b, respectively. It has a configuration in the form of a Perot resonator. As a result, by making the signal light go back and forth many times, as shown in Fig. 2 (B), sharp optical frequency selectivity equivalent to that obtained by passing the signal light isometrically through a multi-stage Mach-Zehnder optical interferometer circuit can be obtained. Realize.

In other words, the optical frequency response of the optical frequency filter of the present invention is based on the sinusoidal characteristic of the Mach-Zehnder optical interferometer circuit whose FSR value is C/n・ΔI1MZ, and the sinusoidal characteristic of the Mach-Zehnder optical interferometer circuit whose FSR value is C/2n-L.
It can also be said that it is the product of the SR value and the peak-like resonance characteristic characteristic of a Fabry-Perot resonator. Here, the role of the thin film heater phase shifter 21 is to tune the transition optical frequency (peak optical frequency f,) of the Mach-Zehnder optical interferometer circuit 16 to a desired value, and the role of the thin film heater phase shifter 20a and The role of 20b is to match the resonant optical frequency of Fabry-Bello resonator 15 to the desired value.

Note that if this resonance condition is not satisfied, the function as an optical frequency filter cannot be obtained, so care must be taken. Since the roles of thin film heater phase shifters 20a and 20b are complementary, it is also possible to omit one of them. Alternatively, in some cases, the thin film heater phase shifter 2
Omit both 0a and 20b and substitute substrate 1
A Bertier element may be disposed below the substrate 1 to control the temperature of the entire substrate 11.

Specific numerical examples of optical waveguide lengths, etc. in the embodiments shown in FIGS. 1(^) and 1(B) are as follows. First, the ΔJ2M2 value is determined by bh Il , zl-2 , taking into account the refractive index value (n ~ 1.456) of the silica-based glass optical waveguide 12 so that FSR-100Gtlz is obtained for the entire optical frequency filter. It was set to 0.06 mm. The total length of the optical waveguide 12 was L=10 mm. The reflecting mirrors 14a and 14b are formed by coating a multilayer film of a 5i02 thin film and a TiO2 thin film on both end surfaces 13a and 13 of the optical waveguide 12.
It was constructed by vapor depositing on b. The reflectance at a wavelength of 1.55 μm was 9596. Thin film heater phase shifter 2
The lengths of 1, 20a and 20b were greater than the ΔAM□ value and could be set to lO+nm, 5+nm and 5mm, respectively. In such a configuration, the temperature increase width ΔT necessary to obtain an optical path length change equivalent to one wavelength in each thin film heater phase shifter was about ΔT−15°C. That is, in this embodiment, the temperature increase range is moderated to an allowable range, compared to the conventional configuration shown in FIG. 4(A), which was about ΔT-80°C.

3 is a plan view of a second embodiment of the optical frequency filter of the present invention. The difference from the first embodiment shown in FIGS. 1(A) and CB) is that the input/output end surface 13a in the first embodiment is
and multilayer interference film reflecting mirrors 14a and 1 provided in 13b.
In the second embodiment, instead of llb, a Fabry-Bello resonator is constructed by loop-shaped light reflection circuits 24a and 24b.

That is, on the substrate ll, the arm optical waveguide 18a
Loop-shaped light reflection circuits 24a and 24b are disposed at both ends of the loop via directional couplers 31a and 31b, respectively. These loop-shaped light reflection circuits 24a and 24b are connected to each optical waveguide 25 via each directional coupler 31a and 31b.
It binds to a and 25b. End face 13 of optical waveguide 25a
A human-powered light Pin is input from a, and output light P is output from the end surface 13b of the optical waveguide 25b. Take out the ut.

The reflectance of the loop-shaped light reflection circuits 24a and 24b as "reflectors" depends on the coupling ratio of the directional couplers 31a and 31b, and when the coupling ratio is 50*, the maximum reflectance (~1004) is obtained, and the binding rate is equal to or 1
It is known that the minimum reflectance (0 frame) is obtained when 0096 (References: 1, D, Miller et
,al,, ”New all-fiber 1ase
r”.

Technical Digest of OFC/1
00C'87. paper WI3).

In FIG. 3, loop-shaped light reflection circuits 24a and 24
Let the loop lengths of b be L and L2, respectively, and the optical waveguide length between the directional couplers 31a and 31b be L3, then the isometric waveguide length as a Fabry-Perot resonator is (Ll+L2 )/2÷shi.

is given by

An asymmetric Mach-Zehnder optical interferometer circuit 16 with an optical waveguide length difference iz is installed in the middle of the optical waveguide 18a having a length L3. This asymmetric Mach-Zehnder optical interferometer circuit 16 consists of two directional couplers 17a and 17b with a coupling ratio of approximately 50* and two arm optical waveguides 18a and 18b with an optical waveguide length difference of Δ-QM2. This is similar to the first embodiment described above in that the thin film heater phase shifter 21 is provided on one arm optical waveguide 18b. Further, a loop-shaped light reflection circuit 24a that occupies a part of the configuration of the Fabry-Bello resonator 15
and 24b are provided with thin film heaters 20a and 20b, respectively, and play the same role as in the first embodiment.

The radius of curvature of the curved portion of the optical waveguide in this example was set to about 5 mm so that bending loss would not occur. The optical waveguide lengths are L, L2-30mm, and L3-25mm.

It was set to ΔftM2・2.013 mm. The length of each of the thin film heater phase shifters 21, 20a and 20b was set to 15 mm.

The coupling ratio of the directional couplers 31a and 31b was set to about 4096, and the reflectance of the loop light reflection circuits 24a and 24b was 94 zero.

With such a configuration, as in the first embodiment, the effective FS
It was possible to obtain an optical frequency filter operation with an R value of 100 G)lx.

The temperature rise width ΔT required to obtain an optical path length change equivalent to one wavelength with each thin-film heater phase shifter is approximately 10°C, and the maximum ΔTJ4
With a temperature increase width of io° C., the selected optical frequency (peak optical frequency f) as an optical frequency filter could be freely moved within the range of the FSR width, that is, a tuning effect could be performed. In this embodiment, the length of the thin film heater is set longer than in the first embodiment, so the required temperature increase width of 6 is smaller. In the conventional configuration shown in Figure 4 (A), the temperature rise range required to tune the optical frequency width corresponding to FSR-100 GHz reached 80 degrees Celsius, but this is a significant improvement. I understand that.

Above, the structure 1, etc. of the optical frequency filter of the present invention has been explained based on a quartz-based single-mode optical waveguide on a silicon substrate. However, the present invention is not limited to quartz-based single-mode optical waveguides, Of course, the present invention can also be applied to optical waveguide systems.

In addition to quartz-based optical waveguide systems, target optical waveguide systems include multi-component glass-based ion diffusion waveguides, LiNb0. Examples include system waveguides. LiNb0. In the case of a system waveguide, it is possible to use a phase shifter that utilizes an electro-optic effect, but if the configuration of the present invention is adopted, the voltage applied to the phase shifter can be reduced, Similar to the thermo-optic effect phase shifter, there is an advantage of not placing an excessive burden on the phase shifter.

The present invention is based on the first aspect. It goes without saying that the configuration is not limited to the optical circuit configuration shown in the second embodiment.
In the Fabry-Bello optical interferometer circuit, an asymmetric Mach
Any configuration may be used as long as the Zehnder optical interferometer circuit is incorporated and the signal light passes through the Mach-Zehnder optical interferometer circuit many times.

[Effects of the Invention] As explained above, the present invention incorporates an asymmetric Mach-Zehnder optical interferometer circuit in the middle of the Fabry-Bello resonator, thereby achieving an optical path length of the Fabry-Bello resonator without shortening it. , an optical frequency filter with a large FSR value can be constructed. Therefore, sufficient space is secured on the optical path to install a phase shifter, such as a thin film heater phase shifter, to turn this optical frequency filter into a tuner pull, which places an excessive burden on the phase shifter. There is an advantage that it is possible to tune to a desired optical frequency value without any interference. Therefore, the optical frequency filter of the present invention is expected to find wide application in the fields of optical frequency multiplexing transmission systems, optical frequency measurement, etc.

[Brief explanation of the drawing]

FIGS. 1(^) and (B) are a plan view and a sectional view taken along line A^' of the first embodiment of the present invention, respectively. FIGS. An optical frequency characteristic diagram for explaining the operating principle of a frequency filter, FIG. 3 is a plan view showing a second embodiment of the present invention, and FIG.
A) and (It) are a plan view showing an example of a Fabry-Bérot optical resonator as a conventional optical frequency filter, and an optical frequency response characteristic diagram thereof, respectively. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Single mode optical waveguide, 3a...Optical waveguide input end, 3b...Optical waveguide output end, 4a, 4b...Reflector, 11...Silicon substrate, 12...Quartz-based single mode optical waveguide, 13a...Waveguide input end, 13b...Waveguide output end, 14a, 14b...Reflector, 15...Fabry-Bello resonator, 16 ...Mach-Zehnder optical interferometer circuit, 17a,
17b... Directional coupler, 18a, 18b... Optical interferometer arm optical waveguide, 19.
...Quartz-based cladding layer, 20a, 20b, 21...Thin film heater phaser, 24a
, 24b...Loop-shaped light reflection circuit, 25a, 25b
- Optical waveguide, 31a, 31b... directional coupler. fr Light engraving lIgaf (A) f "Sen Domoji light (B) #: J Zhou Shiyu" shi [Fill θ shoulder υ support] Bow (special) 1
Main figure 1 figure 2

Claims (1)

[Scope of Claims] 1) a planar substrate; a single mode optical waveguide disposed on the planar substrate and provided with reflecting mirrors at both ends to constitute an optical resonator; An optical frequency filter comprising: an asymmetric Mach-Zehnder optical interferometer circuit inserted in the middle of a waveguide and configured together with a portion of the optical waveguide and a second optical waveguide. 2) The optical frequency filter according to claim 1, wherein the reflecting mirror is a waveguide-type optical reflection circuit composed of a loop-shaped optical waveguide and a directional coupler. 3) A phase shifter for finely adjusting the optical path length and adjusting the filter selection optical frequency is provided on the optical waveguide constituting the Mach-Zehnder optical interferometer and on the optical waveguide constituting the optical resonator. The optical frequency filter according to claim 1 or 2. 4) Each of the optical waveguides is a silica-based single-mode optical waveguide constructed by embedding a core portion made of silica-based glass in a cladding layer made of silica-based glass disposed on the planar substrate, and 4. The optical frequency filter according to claim 3, wherein the filter comprises a thin film heater formed on the cladding layer above the core portion.