JPH07333447A - Optical signal processing circuit - Google Patents

Abstract

PURPOSE:To realize an optical equalizer which compensates dispersion of optical fibers and the array waveguide diffraction gratings having flat optical frequency characteristics with each of respective channels by specifying the parameters of array waveguide diffraction grantings. CONSTITUTION:Signal light of a frequency (f) (wavelength lambda=c/f) is spread by diffraction in the first sectional slab waveguide 22 and is introduced to the channel waveguide array 23 arranged perpendicularly to its diffraction plane when this signal light is made incident on the central part of the channel waveguide 11 for input. At this time, the quantity of the light power to be taken into the respective waveguides of the channel waveguide array 23 depends upon the core opening widths D1 of the respective waveguides. The core opening widths D of the respective waveguides of the channel waveguide array 23 are set at prescribed values at the boundary of the first sectional slab waveguide 22 and the channel waveguide array 23, by which the photoelectric amplitude Bit(n+1) of the (n+1)th (n=0 to N-1) is assigned and the prescribed waveguide length Q(n+1) at about the wavelength lambda of below of the light is adjusted.

Description

Detailed Description of the Invention

[0001]

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

[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 (modulation frequency) f 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. When this distortion increases,
The transmission capacity or transmission distance of the optical fiber is 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. 9, a dielectric 1 and metal conductors 2 and 3 bonded to both surfaces thereof. The propagation delay time τ increases as the signal frequency f increases as shown in FIG. 10 (propagation speed decreases). Also, the length L of the microstrip line
The proportion increases according to. Thus, the propagation delay characteristics are opposite between the microstrip line and the optical fiber. Therefore, the optical signal propagating through the optical fiber having dispersion can be canceled out by the influence of dispersion in the optical fiber by converting the optical signal into an electric signal and then passing through the microstrip line having a predetermined length L.

Next, a conventional arrayed waveguide diffraction grating having a wavelength demultiplexing function will be described with reference to FIGS. 11 to 13. FIG. 11 is a plan view showing the structure of a conventional arrayed waveguide diffraction grating.

In the figure, a plurality (or one) of input channel waveguides 11 formed on a substrate 10, a first fan-shaped slab waveguide 12, and a plurality of N waveguides which are sequentially lengthened by a waveguide length difference ΔL. This is a configuration in which a channel waveguide array 13 including waveguides, a second fan-shaped slab waveguide 14, and a plurality of output channel waveguides 15 are sequentially connected.

FIG. 12 is an enlarged view showing the structure in the vicinity of the first fan-shaped slab waveguide 12. The same applies to the second fan-shaped slab waveguide 14. In the figure, R is the radius of curvature of the first fan-shaped slab waveguide 12, 2a is the core width of each waveguide of the input channel waveguide 11 and the channel waveguide array 13, and U is each waveguide of the input channel waveguide 11. The core opening width of the waveguide, s ₁ is the input channel waveguide 11
, The waveguide spacing at the slab waveguide boundary, D is the core opening width of each waveguide of the channel waveguide array 13, s ₂ is the waveguide spacing at the slab waveguide boundary of the channel waveguide array 13,
d ₁ and d ₂ indicate the length of each tapered waveguide portion. Here, U and D are constant.

In such a structure, the light incident from the predetermined input channel waveguide 11 spreads by diffraction in the first fan-shaped slab waveguide 12, and the channel waveguide array 13 arranged perpendicular to the diffractive surface. Be led to.
In the channel waveguide array 13, each waveguide has a waveguide length difference Δ.
Since it becomes longer in L, it propagates through each waveguide and
The light reaching the fan-shaped slab waveguide 14 has a waveguide length difference Δ
There is a phase difference corresponding to L. Since this phase difference varies depending on the optical frequency, when the light is focused on the input end of the output channel waveguide 15 by the lens effect of the second fan-shaped slab waveguide 14, 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 15 is selected according to the frequency of the light incident from the input channel waveguide 11. To do. In the conventional arrayed waveguide diffraction grating, as shown in FIG.
A parabolic optical frequency characteristic is obtained in the vicinity of the center frequency (here, 100 GHz interval) corresponding to each of the waveguides.

[0009]

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 f
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.

Further, the conventional arrayed-waveguide diffraction grating has a parabolic optical frequency characteristic as shown in FIG. 13 and has a narrow 3 dB bandwidth of 27 GHz. Therefore, when the wavelength of the light incident on the input channel waveguide 11 changes from its center wavelength, the loss of the light emitted to a predetermined channel of the output channel waveguide 15 increases significantly, and There was a problem of degrading crosstalk.

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

[0012]

According to the optical signal processing circuit of the present invention, the core openings of the respective waveguides of the channel waveguide array at the boundary between the first fan-shaped slab waveguide and the channel waveguide array are respectively predetermined. Has a width. Further, each of the waveguides of the channel waveguide array, which is sequentially lengthened by a predetermined waveguide length difference, has a length obtained by adding or subtracting a predetermined waveguide length equal to or less than the wavelength of the signal light.

[0013]

The optical electric field distribution and phase of each waveguide of the channel waveguide array forming the arrayed waveguide diffraction grating are adjusted by adjusting the core opening width of each waveguide and a predetermined waveguide length equal to or less than the wavelength of the signal light. It can be set according to the length of each waveguide.

In the optical signal processing circuit of the present invention, the core opening width and length of each waveguide of the channel waveguide array are adjusted based on this principle. Thereby, the optical electric field distribution and phase of the channel waveguide array can be controlled, and the optical frequency characteristic in each channel of the output channel waveguide can be controlled. For example, it is possible to realize an optical frequency characteristic having a sign opposite to that of the dispersion characteristic of the optical fiber. Further, it is possible to realize an arrayed waveguide diffraction grating having a flat optical frequency characteristic for each channel.

[0015]

1 is a plan view showing the structure of an optical signal processing circuit according to the present invention. In the figure, a plurality (or one) of input channel waveguides 11 formed on a substrate 10 and a first
Fan-shaped slab waveguide 22 and a channel waveguide array 2 composed of a plurality of N waveguides which are sequentially lengthened by a predetermined waveguide length difference.
3, the second fan-shaped slab waveguide 14 and a plurality of output channel waveguides 15 are sequentially connected. The basic configuration is the same as that of the conventional arrayed waveguide diffraction grating shown in FIG. In the present invention, the first fan-shaped slab waveguide 22
And the channel waveguide 23 is different from the conventional one.

FIG. 2 is an enlarged view showing the structure in the vicinity of the first fan-shaped slab waveguide 22. The structure in the vicinity of the second fan-shaped slab waveguide 14 is the same as that of the conventional first fan-shaped slab waveguide 12 shown in FIG.

In the figure, R is the radius of curvature of the first fan-shaped slab waveguide 22, 2a is the core width of each waveguide of the input channel waveguide 11 and the channel waveguide array 23, and U is the input channel waveguide 11. Core opening width, s ₁ is the waveguide spacing at the slab waveguide boundary of the input channel waveguide 11, D _i is the core of the i-th (i is 1 to N) waveguide from one end of the channel waveguide array 23. The opening width, s ₂ is the waveguide spacing at the slab waveguide boundary of the channel waveguide array 23,
d ₁ and d ₂ indicate the length of each tapered waveguide portion. Here, U is constant, but D _i is different for each waveguide.

In this embodiment, the input channel waveguide 11 is used.
It is assumed that signal light of frequency f (wavelength λ = c / f) is incident on the center port of the. The incident signal light spreads by diffraction in the first fan-shaped slab waveguide 22 and is guided to the channel waveguide array 23 arranged perpendicularly to the diffraction surface. At this time, the amount of optical power taken into each waveguide of the channel waveguide array 23 is determined by the core opening width D of each waveguide.
depends on _i . Now, let the optical electric field amplitude of the i-th (i is 1 to N) waveguide be Bit (i) (real number). The channel waveguide array 23 is configured such that each waveguide is sequentially lengthened with a waveguide length difference ΔL from the inside in FIG. 1 and from the right in FIG. In addition to this, a predetermined waveguide length Q (i) having a wavelength of about λ or less is added to or subtracted from the length of the i-th waveguide.

Here, when the length of the rightmost waveguide (i = 1) is set to L _C , the phase of the light when it exits the second fan-shaped slab waveguide 14 through the i-th waveguide. φ _i is expressed as φ _i = β _C {L _C + (i−1) ΔL + Q (i)} (1). However, β _C is the propagation constant of the waveguide.
The light that has entered the second slab waveguide 14 from the i-th waveguide undergoes multiple interference and is emitted to the port (the central port of the output channel waveguide 15 in this embodiment) according to the frequency f of the light. . The electric field amplitude G (f) of the emitted light is

[0020]

[Equation 1]

Is represented as Assuming that the diffraction order of the arrayed waveguide diffraction grating is m _FDM , the following relationship holds: m _FDM = n _C ΔL / λ ₀ = n _C ΔLf ₀ / c (3) However, n _C = β _C / k (4), and λ ₀ and f ₀ are the center wavelength and center frequency of the signal light, respectively.

Further, between the frequency band (Free Spectral Range: FSR) W of the arrayed-waveguide diffraction grating and the diffraction order m _FDM , the relationship of W = f ₀ / m _FDM (5) is established. Here, by discretizing the optical frequency within the frequency band of the array waveguide diffraction grating _{f = f S = f 0 +} sW / N (s = -N / 2 ~ N / 2-1) ... expressed as (6). At this time, from equations (3), (4), (5), and (6), β _C ΔL
The s-th component of is β _C (s) ΔL = 2π (m _FDM + s / N) (7) If equation (1) is rewritten using this, φ _i (s) = β _C (s) L _C + (i-1) 2π (m _FDM + s / N) + β _C (s) Q (i) (8) ). When the s-th component of the electric field amplitude G (f) of the emitted light is calculated using Equation (8) and Equation (2),

[0023]

[Equation 2]

It is expressed as follows. However, Δf = W / N. Here, substituting n = i-1 (n = 0 to N-1),
Equation (9) is

[0025]

[Equation 3]

[0026] However, since L _C >> Q (n + 1), β _C (s) Q (n + 1) is set as β _C (0) Q (n + 1). Here, when g (n) = Bit (n + 1) exp {-jβ _C (O) Q (n + 1)} (11) is set, the equation (10) becomes

[0027]

[Equation 4]

[0028] This equation represents the discrete Fourier transform relationship between g (n) and G (sΔf). That is,
First fan-shaped slab waveguide 22 and channel waveguide array 2
At the boundary with 3, the core opening width of each waveguide of the channel waveguide array 23 is set to a predetermined value and the (n + 1) th (n =
(0 + 1) by specifying the optical field amplitude Bit (n + 1) of 0 to N-1) and adjusting the predetermined waveguide length Q (n + 1) that is less than or equal to the wavelength λ of the light. Adjust the phase of the second waveguide. As a result, the predetermined complex amplitude coefficient g (n) can be realized, and the desired optical frequency characteristic G (sΔf) can be obtained by the equation (12).

On the contrary, the desired optical frequency characteristic G (sΔ
f) has already been given,

[0030]

[Equation 5]

The inverse complex Fourier transform of gives the complex amplitude coefficient g (n). And the (n + 1) th (n = 0 to N-
The optical electric field amplitude Bit (n + 1) of 1) is given as the absolute value of the complex amplitude coefficient g (n) from the equation (11), and the waveguide length Q (n + 1) that is adjusted in the waveguide is It is obtained from the phase term of the complex amplitude coefficient g (n). Thus, the core opening width D of each waveguide of the channel waveguide array 23 at the boundary between the first fan-shaped slab waveguide 22 and the channel waveguide array 23 is
_{n + 1} and the waveguide length Q (n + 1) to be adjusted are determined. The above is a general description of the optical signal processing circuit of the present invention as an optical frequency filter.

(First Embodiment) As a first embodiment of the optical signal processing circuit according to the present invention, a specific example in which the optical signal processing circuit is used in an optical equalizer will be described below.

First, the frequency response H (ω) of the optical fiber is given by H (ω) = H ₀ exp {-j (β ″ L / 2) (ω−ω ₀ ) ² } (14). However, β ″ = d ² β / dω ² , ω ₀ is the central angular frequency of light, L is the fiber length, and H ₀ is a constant.
The relation of β ″ = (λ ₀ ² / 2πc) σ (15) holds between the dispersion σ and β ″ of the optical fiber. Where c is the speed of light in vacuum, λ ₀
= 2πc / ω ₀ .

When the unit of the wavelength λ ₀ is μm, the unit of dispersion σ of the optical fiber is ps / km · nm, and the unit of the fiber length L is km, p = π · 10 ⁻⁵ · λ ₀ ² σL / 3 (16), the frequency response H (ω) of the optical fiber is expressed as H (ω) = H ₀ exp {-jp (f−f ₀ ) ² } (17). However, the unit of optical frequency f and f ₀ is GHz
Is. From this, the signal delay time t _f of the optical fiber is

[0035]

[Equation 6]

Is given by Therefore, the optical frequency characteristics G (sΔf) is G ₀ of the optical signal processing circuit of the present invention as a _{constant, G (sΔf) = G 0} exp {jp (f s -f 0) 2} = G 0 exp {jp ( sΔf) ² } (19), the dispersion characteristics of the optical fiber (equation (14) or (1)
An optical equalizer that compensates for 7)) can be realized.

The specific design of the optical equalizer is expressed by the equation (19) and the equation (13).
By substituting into

[0038]

[Equation 7]

The complex amplitude coefficient g (n) is obtained by the inverse discrete Fourier transform of. As described above, the (n + 1) th (n =
The optical electric field amplitude Bit (n + 1) of 0 to N-1) is given as an absolute value of the complex amplitude coefficient g (n) from the equation (11), and the waveguide length Q (n + 1) is adjusted to the waveguide. ) Is obtained from the phase term of the complex amplitude coefficient g (n). In this way, the core opening width D _{n + 1} of each waveguide of the channel waveguide array 23 at the boundary between the first fan-shaped slab waveguide 22 and the channel waveguide array 23.
The waveguide length Q (n + 1) to be adjusted is determined.

In the arrayed waveguide diffraction grating of this embodiment, λ ₀ = 1.55 μm, N = 128, R = 5.63 mm, ΔL =
1.03749 mm, 2a = 7 μm (core thickness 2t = 6 μm, relative refractive index difference Δ = 0.75%), U = 7 μm, d ₁ = 450 μm,
s ₁ = 50 μm, D ₀ = 12 μm, d ₂ = 750 μm, s ₂ =
Assuming 15 μm, n _C = 1.4507, m _FDM = 971, W =
It becomes 200 GHz and Δf = 1.56 GHz.

With this arrayed waveguide diffraction grating, λ ₀ =
1.55 μm, dispersion σ = −10 ps / km · nm, length L = 10
To compensate (equalize) the dispersion of a 0 km optical fiber,
G (n) is calculated according to (20), and the i (= n + 1) th (i =
The optical field amplitude Bit (i) of 1 to N, n = 0 to N-1) and the length of the waveguide Q (i) to be adjusted are obtained.

FIG. 3 shows the distribution of the optical field amplitude Bit (i),
FIG. 4 shows the distribution of the excess optical path length Q (i) / λ _{0 in which} the adjustable waveguide length Q (i) is normalized by the wavelength. I at the boundary between the first sector slab waveguide 22 and the channel waveguide array 23
The core opening width D _i of the th waveguide is determined as follows. The maximum value of Bit (i) (i = 38th in the case of FIG. 3) is defined as Bmax, and the core opening width corresponding to this is defined as Dmax. That is, in the case of FIG. 3, Dmax = D ₃₈ . Since the core aperture width and the light intensity (square of the optical electric field intensity) propagating in the channel waveguide array are proportional,

[0043]

[Equation 8]

The following relationship holds. Therefore, the core opening width D _i of the i-th waveguide is

[0045]

[Equation 9]

Is given by In formula (22), Dmax = D
₀ = 12 μm, the core opening width D _i of the i-th waveguide is determined, and a mask is produced using the parameters of the above-described arrayed-waveguide diffraction grating. A signal processing circuit was produced.

The manufacturing procedure will be described below. A SiO ₂ lower clad layer was 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 was deposited, and the glass was transparentized in an electric furnace. Next, the core layer was etched using a pattern based on the above design, and an optical waveguide portion was produced. Finally, again SiO ₂
The upper cladding layer was deposited. FIG. 5 shows the measurement result of the phase characteristics of the optical equalizer manufactured in this way.

In FIG. 5, the solid line shows the phase characteristic of the produced optical equalizer. The broken line is the inverse sign of the phase characteristic of optical fiber with dispersion σ = −10 (ps / km ・ nm) and length L = 100 (km) (p = −0.0252 (GHz) ⁻² in Eq. (17)). Show the characteristics. That is, it is the phase characteristic required for the equalizer. This measurement results show that it is possible to equalize accurately dispersion of the optical fiber in the range of 50GHz of _{f = f 0 -25~f 0 +25 (} GHz).

(Second Embodiment) Next, as a second embodiment of the optical signal processing circuit of the present invention, a configuration in the case of being used as an arrayed waveguide diffraction grating having flat optical frequency characteristics will be described.

The basic structure is the same as that used as an optical equalizer. However, the first fan-shaped slab waveguide 22
And the core opening width D _i of each waveguide of the channel waveguide array 23 at the boundary with
The value of (i) is different.

In the arrayed waveguide diffraction grating of this embodiment, λ ₀ = 1.55 μm, N = 128, R = 5.63 mm, ΔL =
254.3 μm, 2a = 7 μm (core thickness 2t = 6 μm, relative refractive index difference Δ = 0.75%), U = 7 μm, d ₁ = 450 μm, s
₁ = 50 μm, D ₀ = 12 μm, d ₂ = 750 μm, s ₂ = 15 μm
When m, n _C = 1.4507, m _FDM = 238, W = 813.2
GHz, Δf = 6.35 GHz.

With this arrayed waveguide diffraction grating, λ ₀ =
To achieve a flat optical frequency characteristic at 1.55 μm, use the formula
In (13),

[0053]

[Equation 10]

Here, g (n) is obtained, and the optical field amplitude Bit (i) of the i (= n + 1) th (i = 1 to N, n = 0 to N-1) and the waveguide length Q (i) to be adjusted. ). FIG. 6 shows the distribution of the optical field amplitude Bit (i), and FIG. 7 shows the adjustable waveguide length Q.
The distribution of the excess optical path length Q (i) / λ _{g obtained} by normalizing (i) with the wavelength λ _g (= λ ₀ / n _C ) in the waveguide is shown. The core opening width D _i of the i-th waveguide at the boundary between the first fan-shaped slab waveguide 22 and the channel waveguide array 23 was determined as Dmax = 12 μm in the equation (22). Such an arrayed waveguide diffraction grating can be manufactured in the same manner as in the case of the optical equalizer. The measurement result of the optical frequency characteristic is shown in FIG.

In FIG. 8, the output channel waveguide 15 is shown.
Can realize flat optical frequency characteristics in the vicinity of the center frequency (here, 100 GHz interval) corresponding to each waveguide, and 3 dB
Bandwidth has been expanded from the conventional 27 GHz to 60 GHz. That is, the 3 dB bandwidth can be significantly increased without deteriorating crosstalk to adjacent channels.

[0056]

As described above, the optical signal processing circuit of the present invention can realize an arbitrary propagation delay characteristic by appropriately selecting the parameters of the arrayed waveguide diffraction grating. 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.

By appropriately selecting the parameters of the arrayed waveguide diffraction grating, the 3 dB bandwidth can be greatly increased without deteriorating the crosstalk to the adjacent signal channels. 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. This allows
The tolerance of the design of the wavelength division routing system or the like can be increased.

[Brief description of drawings]

FIG. 1 is a plan view showing the configuration of an optical signal processing circuit of the present invention.

FIG. 2 is an enlarged view showing a structure in the vicinity of a first fan-shaped slab waveguide 22.

FIG. 3 is an optical field amplitude Bit when used as an optical equalizer.
The figure which shows the distribution of (i).

FIG. 4 is an excess optical path length Q (i) when used as an optical equalizer.
The figure which shows the distribution of / (lambda) ₀ .

FIG. 5 is a diagram showing measurement results of phase characteristics of an optical equalizer.

FIG. 6 is a diagram showing a distribution of an optical electric field amplitude Bit (i) when used as an arrayed waveguide diffraction grating.

FIG. 7 is a diagram showing a distribution of excess optical path length Q (i) / λ _g when it is used as an arrayed waveguide diffraction grating.

FIG. 8 is a diagram showing measurement results of optical frequency characteristics of an arrayed waveguide diffraction grating.

FIG. 9 is a diagram showing a configuration of a conventional equalizer.

FIG. 10 is a diagram showing a propagation delay characteristic of a conventional equalizer.

FIG. 11 is a plan view showing a configuration of a conventional arrayed waveguide diffraction grating.

FIG. 12 is an enlarged view showing a structure in the vicinity of a first fan-shaped slab waveguide 12 (second fan-shaped slab waveguide 14).

FIG. 13 is a diagram showing optical frequency characteristics of a conventional arrayed-waveguide diffraction grating.

[Explanation of symbols]

 10, 20 substrate 11 input channel waveguide 12, 22 first fan-shaped slab waveguide 13, 23 channel waveguide array 14 second fan-shaped slab waveguide 15 output channel waveguide

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 optical signal having 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. In the processing circuit, the core opening of each waveguide of the channel waveguide array at the boundary between the first fan-shaped slab waveguide and the channel waveguide array has a predetermined width, An optical signal processing circuit, wherein each of the waveguides of the channel waveguide array, which becomes longer in sequence, has a length obtained by adjusting a predetermined waveguide length equal to or less than a wavelength of signal light.