JPH09133826A - Light signal processing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the light signal processing circuit used as an optical equalizer which compensates dispersion of an optical fiber or an array waveguide diffraction grating having flat passing band characteristics for each channel. SOLUTION: Two array waveguide diffraction gratings are arrange symmetrically about an axis across a spatial filter 7 which varies the amplitude and phase transmissivity of light. According to the waveguide parameters of those two array waveguide diffraction gratings and the amplitude and phase transmissivity of the spatial filter 7, the optical equalizer which has specific propagation delay characteristics or array waveguide diffraction grating multiplexer demultiplexer which has flat passing band characteristics is constituted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical equalizer for waveform-shaping distortion generated in an optical signal due to dispersion of optical fibers,
Alternatively, the present invention relates to an optical signal processing circuit having a predetermined optical frequency filter characteristic used as an arrayed waveguide diffraction grating having a wavelength demultiplexing function.

[0002]

2. Description of the Related Art Many existing optical fibers have a wavelength of 1.3 μm.
Has zero dispersion and has the property that the loss becomes minimum at the wavelength of 1.55 μm. When an optical signal with a wavelength of 1.55 μm is incident on this optical fiber, the propagation delay time τ decreases (the propagation speed increases) as the optical signal frequency ν increases due to the dispersion of the optical fiber. Therefore, the waveform of the optical signal propagating through this optical fiber is distorted according to the spread of the wavelength spectrum. If this distortion increases, the transmission capacity or transmission distance of the optical fiber will be limited.

The equalizer compensates for such dispersion of the optical fiber and shapes the waveform of the optical signal. As a conventional equalizer, a microstrip line that converts an optical signal into an electric signal and uses it is known. The structure is, as shown in FIG. 4, a dielectric 51 and metal conductors 52 and 53 joined to both surfaces thereof. The propagation delay time τ increases as the signal frequency ν increases as shown in FIG. 5 (propagation speed decreases). In addition, the ratio increases according to the length L of the microstrip line. Thus, the propagation delay characteristics are opposite between the microstrip line and the optical fiber. Therefore, an optical signal propagating through an optical fiber having dispersion is converted into an electric signal and then converted into a predetermined length L.
The influence of dispersion in the optical fiber can be canceled out by passing the microstrip line of.

Next, a conventional arrayed waveguide diffraction grating having a wavelength demultiplexing function will be described. FIG. 6 shows a configuration of a conventional arrayed waveguide diffraction grating. The conventional arrayed-waveguide diffraction grating includes an input channel waveguide 61, a channel waveguide array 62 configured to be sequentially elongated with a predetermined waveguide length difference ΔL, an output channel waveguide 63, and an input channel waveguide. 61, a first fan-shaped slab waveguide 64 for connecting the channel waveguide array 62, and the channel waveguide array 6
The second fan-shaped slab waveguide 65 is formed on the substrate 60 to connect the 2 and the output channel waveguide 63.

Light incident from a predetermined input channel waveguide 61 spreads by diffraction in the first fan-shaped slab waveguide 64 and is guided to a channel waveguide array 62 arranged perpendicularly to the diffractive surface. In the channel waveguide array 62, since the respective waveguides are sequentially lengthened by the waveguide length difference ΔL, the light having propagated through the respective waveguides and reaching the second fan-shaped slab waveguide 65 has a waveguide length difference ΔL. There is a phase difference corresponding to. Since this phase difference varies depending on the optical frequency, when the light is focused on the input end of the output channel waveguide 63 by the lens effect of the second fan-shaped slab waveguide 65, it is focused on different positions for each optical frequency.

The arrayed waveguide diffraction grating operates as an optical demultiplexer in which the waveguide of the output channel waveguide 63 is selected according to the frequency of the light incident from the input channel waveguide 61. To do. In the conventional arrayed-waveguide diffraction grating, as shown in FIG. 7, the center frequency (200 GHz (1.
A parabolic pass band characteristic is obtained in the vicinity of (6 nm) interval).

[0007]

In the conventional equalizer using a microstrip line, it is necessary to temporarily convert an optical signal into an electric signal for waveform shaping, and it cannot be used in an all-optical repeater system. Furthermore, the signal frequency ν
Since the conductor loss of the microstrip line increases with increasing, it is difficult to increase both the transmission capacity and the transmission distance of the optical fiber even if the waveform shaping of the optical signal is performed.

The conventional arrayed waveguide diffraction grating has a parabolic pass band characteristic as shown in FIG. Therefore, when the wavelength of the light incident on the input channel waveguide 61 fluctuates from its center wavelength, the loss of the light emitted to a predetermined channel of the output channel waveguide 63 greatly increases, and There was a problem of degrading crosstalk.

The present invention realizes an optical equalizer for compensating for dispersion of an optical fiber, and an arrayed waveguide diffraction grating having a flat pass band characteristic for each channel, and realizes large capacity / long-distance optical communication and wavelength division. An object is to provide an optical signal processing circuit suitable for routing.

[0010]

In the optical signal processing circuit of the present invention, two arrayed waveguide diffraction gratings are arranged line-symmetrically via a spatial filter that changes the amplitude and phase transmittance of light. Depending on the waveguide parameters of the two arrayed waveguide diffraction gratings and the amplitude and phase transmittance of the spatial filter,
An optical equalizer having a predetermined propagation delay characteristic or an arrayed waveguide diffraction grating multiplexer / demultiplexer having a flat pass band characteristic is configured.

[0011]

FIG. 1 shows an embodiment of an optical signal processing circuit of the present invention. The optical signal processing circuit of the present invention comprises an input channel waveguide 1, a channel waveguide array 2 configured to be sequentially elongated with a predetermined waveguide length difference ΔL, an input channel waveguide 1 and a channel waveguide array 2. And a pair of array waveguide diffraction gratings each composed of a first sector slab waveguide 3 for connecting to and a second sector slab waveguide 4 connected to the other end of the channel waveguide array 2 Of the fan-shaped slab waveguide 4 on the substrate 6 with respect to the line symmetry axis 8 passing through the end midpoint (hereinafter referred to as "line symmetry center 5") to which the channel waveguide array is not connected. Are arranged in line symmetry.

That is, 11 is an input channel waveguide 1.
Is a channel waveguide array for output, 22 is a channel waveguide array configured to be sequentially elongated by a predetermined waveguide length difference ΔL corresponding to the channel waveguide array 2, and 33 is a fan-shaped slab waveguide. Reference numeral 44 denotes a fan-shaped slab waveguide that connects the output channel waveguide 11 and the channel waveguide array 22 corresponding to the waveguide 3, and 44 is connected to the other end of the channel waveguide array 22 corresponding to the fan-shaped slab waveguide 4. It is a fan-shaped slab waveguide. The fan-shaped slab waveguide 4 and the fan-shaped slab waveguide 44 face each other with the line symmetry center 5 therebetween.

Further, a spatial filter 7 for changing the amplitude and phase transmittance of light is arranged in a direction passing through the line symmetry center 5 and perpendicular to the light propagation direction in the fan-shaped slab waveguides 4, 44. Now, it is assumed that the number of input channel waveguides 1 and the number of output channel waveguides 11 are each eight. The input port numbers of the input channel waveguide 1 are # 1 to # 8 in order from the top of FIG.
Output port numbers of # 1 to # 8 from the top of FIG. At this time, in the light incident from each input port,
The wavelengths of light passing through the axisymmetric center 5 are as shown in Table 1.

[0014]

[Table 1]

Here, λ ₀ is the center wavelength of light, Δλ is the channel wavelength interval, and the channel frequency interval is given by Δν = cΔλ / λ ₀ ^2, where c is the speed of light. Further, the light of each wavelength shown in Table 1 passing through the line symmetry center 5 is emitted to the output port with the number shown in Table 2.

[0016]

[Table 2]

Therefore, by disposing the spatial filter 7 for changing the amplitude and the phase transmittance of light in the vicinity of the line symmetry center 5, a predetermined transmission characteristic for each wavelength with respect to the light incident from each input port is provided. Can be given.
The relationship between the shift amount δx of the light beam spot with respect to the direction perpendicular to the light propagation direction in the fan-shaped slab waveguides 4 and 44 passing through the axisymmetric center 5 and the frequency change amount δν (wavelength change amount δλ) of light is δx / δν = f · δL / (d · ν ₀ ) ... (1-1) δx / δλ = −f · δL / (d · λ ₀ ) ... (1-2) Here, d (= 25 μm) is the waveguide spacing between the channel waveguide arrays 2 and 22 at the boundary between the channel waveguide array 2 and the sector slab waveguide 4, and between the sector slab waveguide 44 and the channel waveguide array 22. . Further, ν ₀ (= c / λ ₀ ) is the center frequency of light.

Now, the core width of the input channel waveguide 1, the channel waveguide arrays 2 and 22, and the output channel waveguide 11 is 2a = 7 μm, the core thickness is 2t = 7 μm, and the relative refractive index difference is Δ = 0.75%. And fan-shaped slab waveguides 3, 4, 33,
The radius of curvature of 44 is f = 11.52 nm, and the waveguide length difference between the channel waveguide arrays 2 and 22 is ΔL = 63 μm. In the case of this parameter, δx / δν = 0.15 [μm / GHz] (2-1) δx / δλ = 20/3 [GHz / μm] (2-2) As shown in Tables 1 and 2, since the center wavelength (frequency) interval of each light passing through the line symmetry center 5 is Δλ (Δν), the optical pass band wavelength (frequency) width of the spatial filter 7 is
± Δλ / 2 to avoid crosstalk to other channels
It must be (± Δν / 2). For this, the formula
From (1), the width of the light passage slit of the spatial filter 7 is centered on the center 5

[0019]

(Equation 1)

It can be seen that it must be in the range of.
In the case of the above parameters, the light passing slit width of the spatial filter 7 is -15 [μm] ≤ x ≤ 15 [μm] (4) from the line symmetry center 5 according to the equations (2) and (3). It becomes a range. The above is a general description of the optical signal processing circuit of the present invention as an optical frequency filter.

[0021]

Embodiment (First Embodiment: Optical Equalizer) As a first embodiment of an optical signal processing circuit of the present invention, a configuration when used in an optical equalizer will be described below. The frequency response H (ω) of many existing optical fibers is given by H (ω) = H ₀ exp {-j (β ″ L / 2) (ω−ω ₀ ) ² } (5) , Β is the propagation constant of light in the optical fiber, β ″ = d ² β / dω ² , ω ₀ (= 2πν ₀ ) is the central angular frequency of light, ω (= 2πν) is the angular frequency, L is the fiber length, H ₀ is a constant. The relationship of β ″ = (λ ₀ ² / 2πc) σ (6) holds between the dispersion σ and β ″ of the optical fiber.

When the unit of wavelength λ ₀ is μm, the unit of dispersion σ of the optical fiber is ps / km · nm, and the unit of fiber length L is km, p = π · 10 ⁻⁵ · λ ₀ ² σL / 3 (7), the frequency response H (ν) of the optical fiber is expressed as H (ν) = H ₀ exp {-jp (ν-ν ₀ ) ² } (8). However, the units of the optical frequencies ν and ν ₀ are G
Hz. From this, the signal delay time t of the optical fiber
_f is

[0023]

(Equation 2)

Is given by Here, the complex (amplitude / phase) transmittance s (x) of the spatial filter 7 arranged in the range of −15 μm to +15 μm about the line symmetry center 5 is s ₀ ,
When s (x) = s ₀ exp (+ jqx ² ) ... (10) with q as a constant, for example, for light from the input port # 4 to the output port # 5, equations (1) and According to Tables 1 and 2, x = (f · ΔL / (d · ν ₀ )) (ν−ν ₀ ) ... (11). Therefore, the phase characteristic G ([nu) are G ₀ of the light that has passed through the spatial filter 7 as a _{constant, G (ν) = G 0} exp {+ jP (ν-ν 0) 2} ... (12) P = q ( f · ΔL / (d · ν ₀ )) ² (13) In the equations (10) and (13), if q = p (dν ₀ / (fΔL)) ² (14), then P = p, and the phase characteristic G of the light passing through the spatial filter 7 (ν) is G (ν) = G ₀ exp {+ jp (ν−ν ₀ ) ² } (15) From this, the signal delay time t _eq of the optical equalizer of this embodiment is

[0025]

(Equation 3)

## EQU1 ## Comparing with the signal delay time t _f of the optical fiber of the equation (9), the signal delay time t _eq of the optical equalizer of the equation (16) has the opposite characteristic. It can be seen that the dispersion of the optical fiber can be compensated (equalized). The optical equalizer of this embodiment can be manufactured using a silica-based optical waveguide. The manufacturing procedure is shown below.

First, a SiO ₂ lower clad layer is deposited on a silicon substrate by a flame deposition method, and then a core layer of SiO ₂ glass doped with GeO ₂ as a dopant is deposited, followed by transparent vitrification in an electric furnace. Next, the core layer is etched using the pattern shown in FIG. 1 based on the above design to produce an optical waveguide portion. Finally, the SiO ₂ upper clad layer is deposited again. Next, a gap for inserting the spatial filter 7 is formed by etching, and the spatial filter 7 having the complex transmittance given by the equations (10) and (14) is inserted into the gap.

FIG. 2 shows the measurement result of the phase characteristics of the optical equalizer manufactured in this way. The solid line shows the phase characteristic of the manufactured optical equalizer. The broken line shows the inverse sign of the phase characteristic of the optical fiber with dispersion σ = −10 [ps / km · nm] and length L = 10 [km] (p = −0.0252 (GHz) ⁻² in equation (8)). Show the characteristics. That is, it is an ideal phase characteristic required for the equalizer.
The result of this measurement is 120 for ν = ν ₀ -60 to ν ₀ +60 [GHz].
It can be seen that the dispersion of the optical fiber can be accurately equalized in the range of [GHz].

(Second Embodiment: Array Waveguide Diffraction Grating with Flat Passband Characteristics) Next, as an second embodiment of the optical signal processing circuit of the present invention, an arrayed waveguide diffraction grating with flat passband characteristics is used. The configuration when used will be described. The basic configuration is the same as when it is used as an optical equalizer. Here, the complex transmittance s (x) of the spatial filter 7 is
Is

[0030]

(Equation 4)

It is assumed that At this time, according to the equations (2), (17) and Tables 1 and 2, the light is incident on the input port #i (i = 1 to 8), passes through the line symmetry center 5, and is output to the output port # (9-i). The transmitted light has a center wavelength λ ₀ + (i
-4) Array waveguide diffraction grating type multiplexer / demultiplexer having a substantially flat pass band characteristic in the range of Δν = ± (20/3) × 12 = ± 80 [GHz] (18) centering on | Δλ | Is obtained.

The arrayed-waveguide diffraction grating of this embodiment has the first
It can be manufactured in the same manner as in the case of the optical equalizer of the embodiment. However, the spatial filter 7 having the complex transmittance given by the equation (17) is inserted. FIG. 3 shows the measurement results of the pass band characteristics of the arrayed waveguide diffraction grating manufactured in this way. As shown in the figure, the passband characteristics are flattened, and the 3 dB bandwidth, which was 113 GHz (Fig. 7) in the conventional arrayed-waveguide diffraction grating, is reduced without deteriorating the crosstalk to the adjacent channel. Expanded to 200 GHz.

[0033]

As described above, in the optical signal processing circuit of the present invention, an arbitrary propagation delay characteristic can be obtained by appropriately selecting the waveguide parameters of the arrayed waveguide diffraction grating and the light passage slit width of the spatial filter. Can be realized.
As a result, it becomes possible to perform waveform shaping for compensating for dispersion of the optical fiber without converting the optical signal into an electric signal, and it is possible to easily realize large-capacity / long-distance optical communication.

Further, by appropriately selecting the waveguide parameter of the arrayed waveguide grating and the light passage slit width of the spatial filter, the 3 dB bandwidth is significantly increased without deteriorating the crosstalk to the adjacent signal channels. Can be made. Therefore, for example, even when the wavelength of the laser light source changes from the center wavelength of each signal channel due to temperature change, it is possible to maintain the predetermined demultiplexing characteristic without increasing the passage loss. As a result, it is possible to increase the design tolerance of the wavelength division routing system and the like.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an optical signal processing circuit of the present invention.

FIG. 2 is a diagram showing measurement results of phase characteristics of an optical equalizer.

FIG. 3 is a diagram showing measurement results of pass band characteristics of an arrayed waveguide diffraction grating.

FIG. 4 is a diagram showing a configuration of a conventional equalizer (microstrip line).

FIG. 5 is a diagram showing a transmission delay characteristic of a conventional equalizer.

FIG. 6 is a diagram showing a configuration of a conventional arrayed-waveguide diffraction grating.

FIG. 7 is a diagram showing measurement results of pass band characteristics of a conventional arrayed waveguide diffraction grating.

[Explanation of symbols]

 1 input channel waveguide 2,22 channel waveguide array 3,4,33,44 fan-shaped slab waveguide 5 line symmetry center 6 substrate 7 spatial filter 8 line symmetry axis 51 dielectric 52,53 metal conductor 60 substrate 61 for input Channel waveguide 62 Channel waveguide array 63 Output channel waveguide 64,65 Fan-shaped slab waveguide

Claims (3)

[Claims] 1. A channel waveguide for input, a channel waveguide array configured such that the lengths of the waveguides sequentially increase with a predetermined waveguide length difference, an input channel waveguide and a channel waveguide array. A pair of arrayed waveguide diffraction gratings each having a first fan-shaped slab waveguide connected to each other and a second fan-shaped slab waveguide connected to the other end of the channel waveguide array. The two arrayed waveguide diffraction gratings are arranged on the substrate in line symmetry with respect to the line symmetry axis passing through the end midpoint where the channel waveguide array of the waveguide is not connected, An optical signal processing circuit, characterized in that a spatial filter for changing the amplitude and phase transmittance of light is arranged in a direction perpendicular to the propagation direction of light in the second fan-shaped slab waveguide. 2. An optical equalizer having a predetermined propagation delay characteristic according to the waveguide parameters of two arrayed waveguide diffraction gratings arranged line-symmetrically on a substrate and the amplitude and phase transmittance of a spatial filter. It is comprised, It is characterized by the above-mentioned.
The optical signal processing circuit according to. 3. An arrayed waveguide diffraction grating having flat passband characteristics according to the waveguide parameters of two arrayed waveguide diffraction gratings arranged line-symmetrically on a substrate and the amplitude and phase transmittance of a spatial filter. The optical signal processing circuit according to claim 1, wherein a multiplexer / demultiplexer is configured.