JP2012230337A - Wavelength selective switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reproduce a desired phase shift function in a wavelength selective switch using a spatial light modulator.SOLUTION: With respect to positional relations among incident signal light, diffracted light signal, and an LCOS being a special phase modulator in a wavelength selective switch, one of two directions orthogonal to each other of a pixel surface of the LCOS is made to coincide with a wavelength dispersion direction of input signal light impinging on the LCOS, and only the other direction is a diffraction direction of the diffracted signal light. In this case, the upper limit of a spot size determined by band limitation is not limited because a channel band is independent of linear dispersion. Meanwhile, the lower limit of the spot size expressed by formula (20) is caused by the fact that a phase modulation function is too coarse for the spot size and a Gaussian beam condensed on the spatial phase modulator is sensitive to a modulation function having a high frequency component, and the lower limit of the spot size is applied to this embodiment because occurring independently of linear dispersion.

Description

  The present invention relates to a wavelength selective switch, and more particularly to a wavelength selective switch applicable to an optical communication system.

  In recent years, the capacity of optical communication has been increasing, but the transmission capacity has been increasing due to the wavelength division multiplexing (WDM) system, and there is a strong demand for increasing the throughput of the path switching function at the node. Yes. Conventionally, such a path switching has been mainly performed by an electric switch after converting a transmitted optical signal into an electric signal. However, the optical switch takes advantage of the characteristics of an optical signal that is high-speed and broadband. For example, a reconfigurable optical add / drop multiplexer (ROADM) system that performs add / drop using an optical signal or the like is introduced. Specifically, an optical signal is added / dropped at each node using a ring type network, and those that do not need to pass through the optical signal as it is, so that the node device is small and has low power consumption. is there. A wavelength selective switch module is required as a device necessary for future development of ROADM, and a wavelength selective switch using a bulk diffraction grating and a spatial phase modulator has been proposed (see Non-Patent Document 1).

G. Baxter et al., “Highly programmable wavelength selective switch based on liquid crystal on silicon switching element,” OFC2005, OTuF2. Finisar DWP 100 WAVELENGTH SELECTIVE SWITCH PRODUCT BRIEF. Nishihara, Koyama "Lightwave Electronics", pp. 72-75, Corona, 1978

  However, the spatial phase modulator in which the phase applying element is converted into a pixel has a finite width, and therefore it is necessary to reproduce the phase shift function on the spatial phase modulator with an appropriate size. In addition, regarding the spot size of the signal light collected on the spatial phase modulator, if the size is too small, the number of overlapping phase imparting elements decreases, and the phase shift function cannot be felt. On the other hand, if the spot size of the signal light collected on the spatial phase modulator is too large, the phase shift function is blurred and an appropriate phase shift cannot be given to the signal light.

  The present invention has been made in view of such problems, and an object thereof is to reproduce a desired phase shift function in a wavelength selective switch using a spatial light modulator.

  In order to achieve such an object, according to a first aspect of the present invention, there is provided a spectral means for demultiplexing wavelength-multiplexed signal light, which is obtained by wavelength-multiplexing signal light of a plurality of wavelengths, for each wavelength, and a phase relative to the light A spatial phase modulator having a plurality of phase-giving elements pixelated with a width X to give a shift, and optical means for condensing the signal light for each wavelength demultiplexed by the spectroscopic means to the spatial phase modulator The first and second directions orthogonal to each other of the phase applying element surface of the spatial phase modulator are the wavelength dispersion direction, and the second direction is In the diffraction direction, the spatial spread of the electric field of the signal light collected on the spatial phase modulator is defined by the spot size w of the Gaussian beam, and the range of the spot size w is limited by the following equation: It is characterized by being.

ここで、ｎ１（ＸＴ，Ｎ）、ｎ２（ＸＴ，Ｎ）及びｎ３（ＸＴ，Ｎ）は、それぞれ所望のクロストーク値ＸＴ及びチャネル帯域内の位相付与素子数Ｎに依存する第１、第２及び第３の下限係数である。

Here, n1 (XT, N), n2 (XT, N) and n3 (XT, N) are the first and second values depending on the desired crosstalk value XT and the number N of phase imparting elements in the channel band, respectively. And the third lower limit coefficient.

  Further, according to a second aspect of the present invention, in the first aspect, the first to third lower limit coefficients are:

It is characterized by being.

  The third aspect of the present invention is characterized in that, in the first or second aspect, w / X ≧ 0.68 is satisfied.

  According to a fourth aspect of the present invention, in the first aspect, the first direction is a chromatic dispersion direction and a diffraction direction, the second direction is a diffraction direction, and the range of the spot size w is as follows. It is limited by the formula.

Here, B ₀ is a frequency interval B of the wavelength multiplexed signal light, and a band in which the dispersion compensation value falls within an allowable range from a desired value, m 1 (XT, N), m 2 (XT, N) and m 3 (XT, N ) Are first, second, and third upper limit coefficients that depend on the desired crosstalk value XT and the number N of phase imparting elements in the channel band, respectively.

  According to a fifth aspect of the present invention, in the fourth aspect, the first to third upper limit coefficients depend only on the crosstalk value XT, respectively, and the first to third upper limit coefficients It is a coefficient that depends on an exponent with respect to the talk value XT.

  Further, according to a sixth aspect of the present invention, in the fourth or fifth aspect, the first to third lower limit coefficients are:

The first to third upper limit coefficients are expressed by the following formulas using a coefficient having an error of ± 5% with respect to a desired crosstalk value XT.

  The seventh aspect of the present invention is characterized in that, in the sixth aspect, w / X ≧ 0.68 is satisfied.

  According to the present invention, when reproducing a phase shift function for obtaining a desired diffraction angle on a spatial phase modulator, distortion of the function due to the effect of discretization of the phase shift function is suppressed, and desired diffracted light in a wide band. It has the advantage that it can be obtained.

It is a conceptual diagram explaining that the sampled phase shift function takes a constant value with the width | variety of a phase provision element. It is a conceptual diagram explaining the amplitude of the Gaussian distribution beam condensed on a spatial phase modulator. It is a figure which shows the example of the spatial frequency spectrum of the light beam of a phase shift function, a zero order holder, and a Gaussian distribution. It is a figure which shows the example of the phase shift function reproduced. It is a figure which shows the example of the phase shift function reproduced. It is a figure explaining the diffraction in the phase surface with a sinusoidal phase error. It is a figure which shows the relationship between the spot size in the sampling number of various phase shift functions, and a crosstalk value. It is a figure which shows the normalization zone | band B / _B0 dependence of the normalization spot size w / X which gives a desired permissible crosstalk value. It is a figure which shows the normalized zone | band B / _B0 dependence of the normalized spot size w / X which gives a desired permissible crosstalk value with respect to the number N of phase imparting elements in various channel bands. It is a block diagram which shows the wavelength selective switch of 1st Example of this invention. In the wavelength selective switch of the 1st Example of this invention, it is a figure which shows the relationship between a spatial phase modulator and the direction of input-output signal light. It is a figure which shows the relationship between a spatial phase modulator and the direction of input-output signal light in the wavelength selective switch of 2nd Example of this invention. It is a figure which shows the crosstalk value of the wavelength selective switch of a 1st Example (pixel width 2 micrometers). It is a figure which shows the crosstalk value of the wavelength selective switch of a 1st Example (pixel width 5 micrometers). It is a figure which shows the crosstalk value of the wavelength selective switch of a 1st Example (pixel width 10 micrometers). It is a figure which shows the crosstalk value of the wavelength selective switch of a 1st Example (pixel width 10 micrometers, the pixel number 100 of one signal light). It is a figure which shows the crosstalk value of the wavelength selective switch of a 1st Example (pixel width 5 micrometers, the pixel number 10 of one signal light). It is a figure which shows the crosstalk value of the wavelength selective switch of a 2nd Example (pixel width 2 micrometers, the number 100 of pixels of one signal light). It is a figure which shows the crosstalk value of the wavelength selective switch of a 2nd Example (pixel width of 5 micrometers, the number of pixels of one signal light 100). It is a figure which shows the crosstalk value of the wavelength selective switch of a 2nd Example (pixel width 10 micrometers, the pixel number 100 of one signal light). It is a figure which shows the crosstalk value of the wavelength selective switch of a 2nd Example (pixel width 5 micrometers, the pixel number 10 of one signal light). It is a figure which shows the crosstalk value of the wavelength selective switch of a 2nd Example (pixel width of 5 micrometers and the pixel number 1000 of one signal light). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (100 pixel number of one signal light, normalized spot size 10). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (100 pixel number of one signal light, normalized spot size 5). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (100 pixels of one signal light, normalized spot size 1.5). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (10 pixels of one signal light, normalized spot size 1). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (10 pixels of one signal light, normalized spot size 0.5). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (10 pixels of one signal light, normalized spot size 0.15). It is a figure which shows the relationship between the spot size of the wavelength selective switch of a 2nd Example, and a zone | band (1000 pixel count of one signal light, normalized spot size 100). It is a figure which shows the relationship between the spot size of the wavelength selective switch of a 2nd Example, and a zone | band (1000 pixel numbers of one signal light, normalized spot size 50). It is a figure which shows the relationship between the spot size of a wavelength selective switch of a 2nd Example, and a zone | band (1000 pixels of one signal light, normalized spot size 15).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, the principle of the wavelength selective switch according to the present invention will be described, and then an embodiment will be described.

(Principle of wavelength selective switch according to the present invention)
The wavelength selective switch according to the present invention is pixelized with a width X that gives a wavelength shift to a wavelength-division-multiplexed signal light obtained by wavelength-multiplexing a plurality of wavelength signal lights, and a phase shift for the light. A wavelength selective switch comprising: a spatial phase modulator having a plurality of phase imparting elements; and an optical means for condensing the signal light for each wavelength demultiplexed by the spectroscopic means on the spatial phase modulator. The main feature is that the spot size w of the signal light for each wavelength focused on the phase modulator is limited. Hereinafter, how the spot size w is limited will be described.

  In order to change the direction of the signal light with the spatial phase modulator, a linear (straight) phase shift function or a sawtooth phase shift function is formed on the spatial phase modulator, mainly in the direction of zero-order diffraction. The signal light is diffracted. The saw-tooth phase shift function can be treated as a function obtained by folding a straight-line phase shift function with a certain phase value, and the direction of the zero-order diffraction is the same, so the straight-line phase shift function and the saw-tooth phase shift function are There is an equivalent relationship. Therefore, a linear phase shift function will be described below.

  By creating a phase shift function with a plurality of pixelated phase imparting elements on the spatial phase modulator, the phase shift function becomes a discretization function. That is, a linear phase shift function ψ (x) = αx (x is a variable indicating a position in one of two orthogonal directions of the two-dimensional spatial phase modulator, and α is a phase shift. Making a constant related to the slope of a straight line as a function means that ψ (x) is sampled at a constant interval X. Here, when a normalized position variable x ′ = x / X is introduced and a normalized phase shift function ψ (x ′) = x ′ is defined,

Can be written. Sampling ψ (x) with period X is equivalent to sampling ψ (x ′) with period 1. A function ψ _s (x ′) obtained by sampling ψ (x ′) at period 1 is

Can be written. In this equation, n is an integer, δ (x ′) is a delta function,

Is a normalized phase shift function. Here, the range of the phase shift function, that is, one signal light in the wavelength-division multiplexed signal light after demultiplexing is out of two orthogonal directions of the two-dimensional beam collected on the spatial phase modulator. If the range of light collection in one direction is | x | ≦ x ₀ and the number of sample points in the range of x> 0 and x <0 is N ₊ and N ₋ , respectively,

It becomes. The total number of samples N is N = N ₊ + N ₋ +1, including the sample point of x = 0. The Fourier transform of ψ _s (x ′) is

  Here, ξ is a spatial angular frequency normalized by 1 / X. Equation (8) can be rewritten as follows using positive numbers k = 1,... When N is an odd number

When N is an even number

Here, s = e ^−jξ is defined. Since s = 1 when ξ = 0, from the sum of natural numbers, when N is an odd number,

When N is an even number

となる。

It becomes.

Further, since the phase within the finite width of the phase applying element is constant, the state of the sampled phase shift function ψ (x ′) has a function form as shown by a solid line in FIG. (X ').) This is equivalent to sampling ψ (x) with a zero-order holder. The Fourier transform H ₀ (ξ) of the zero-order holder of the sampling period is

And its amplitude is

(Here, the definition of sinc (x) = sin (x) / x is used). For this reason, the effect of | H ₀ (ξ) | also affects the spectrum of the phase shift function.

  On the other hand, the spectrum of the beam spot collected on the spatial phase modulator also affects the spectrum of the phase shift function. The amplitude of the Gaussian beam focused on the spatial phase modulator with the spot size w (Fig. 2) is

It can be expressed as. The Fourier transform G (ξ) of g (x ′) is

It becomes.

FIG. 3 shows each amplitude spectrum when N = 31 and w / X = 3 as an example of a phase shift function, a zero-order holder, and a Fourier transform of a Gaussian beam, that is, a spatial spectrum. The amplitude spectrum | Φ (ξ) | of the phase shift function is indicated by a thin line, the amplitude spectrum | H ₀ (ξ) | of the zero-order holder is indicated by a dotted line, and the amplitude spectrum G (ξ) of the Gaussian distribution beam is indicated by a one-dot chain line. The solid line shows the amplitude spectrum | Φ (ξ) × H ₀ (ξ) | of the sampled and zero-order held phase shift function. In order to reproduce a linear function on the spatial phase modulator, the amplitude spectrum of the phase shift function must be zero with | ξ |> π. Further, it is necessary to maintain the shape of | Φ (ξ) | even when | ξ | ≦ π. From FIG. 3, the Gaussian beam acts as a filter and is expected to have an effect of attenuating a high frequency component of | ξ |> π of the amplitude spectrum of the phase shift function. However, since the spectral width of the Gaussian beam changes depending on the spot size w, the high frequency component of | ξ |> π of the amplitude spectrum of the phase shift function is sufficiently attenuated, and the frequency of | ξ | ≦ π The spot size range is limited so as not to attenuate the component as much as possible.

  A phase shift function reproduced on the spatial phase modulator can be obtained by performing inverse Fourier transform on the spectrum after the high frequency component is attenuated by the filter effect of the Gaussian beam. As an example, FIGS. 4 and 5 are reproduced on a spatial phase modulator when N = 31, w / X = 3, X = 10 μm and N = 31, w / X = 0.4, X = 10 μm, respectively. The phase shift function is shown. In both figures, the point ◯ indicates the sampling point of the phase shift function, and the solid line indicates the phase shift function reproduced on the spatial phase modulator. When w / X = 3, the reproduced phase shift function closely matches the original phase shift function, but distortion occurs toward the end of the band. On the other hand, when w / X = 0.4, it can be seen that the reproduced phase shift function oscillates sinusoidally with respect to the position. Therefore, if the spot size is too large, diffraction will not be obtained in the correct direction at the end of the band, and the band will be narrowed. If it is too small, the phase modulation function will be too coarse for the spot size. Therefore, since the Gaussian beam collected on the spatial phase modulator feels a modulation function having a high frequency component, a property appears in a sinusoidal phase shift function.

When the slope of the normalized phase shift function deviates from 1, the diffraction direction deviates from the original direction, and this light becomes crosstalk with other signal light. The relationship between the degree of deviation of the normalized phase shift function from 1 and the crosstalk can be considered as follows. As shown in FIG. 6, when the incident light having an amplitude A ₀ is diffracted by a diffraction surface having a phase shift function with a minute amplitude a and a period c and a sinusoidal phase error, the coupling surface of the fiber and the signal light. The amplitude distribution U (x ₂ ) of the diffracted light in the x ₂ plane is

(See Non-Patent Document 3). Here, J _m (a) is an m-th order Bessel function. Equation (17) indicates that the diffracted light appears in u = ± 2πm / c (m = 0, 1, 2,...), And that each amplitude is proportional to the m-th order Bessel function J _m (a). Show. Since a region having a small crosstalk value (for example, −40 dB) is considered, it can be handled as a ≦ 1. In this case, since J _m (a)> J _{m + 1} (a), it is sufficient to consider a first-order Bessel function as the crosstalk light. Here, assuming that the optical powers of the signal lights of a plurality of wavelengths are equal, the optical power coupled to the optical fiber in the desired direction when there is no sinusoidal phase error is the diffracted light of equation (17). Since it is expressed as the sum of power, the signal light power is

It will be proportional to Therefore, the crosstalk due to the first-order diffracted light is

It becomes. From equation (18), the condition that the crosstalk due to the sinusoidal phase error is XT (dB) or less is

From

It becomes. The band can be estimated from the range of the phase shift function that satisfies the condition expressed by Expression (19).

  Below, the example of the zone | band calculated | required by Formula (19) is shown.

  FIG. 7 is a diagram showing the relationship between the spot size and the crosstalk value in the number of samplings of various phase shift functions, that is, the number N of phase applying elements that give a phase shift to one signal light. In the figure, the vertical axis is normalized by dividing the spot size by the number of phase applying elements. As is clear from the figure, the relationship between the normalized spot size and the crosstalk value hardly depends on N. The relationship between the normalized spot size and the crosstalk value is approximately expressed by a quadratic function.

It can be expressed as. Further, since the crosstalk value increases as the normalized spot size decreases, Equation (20) gives the lower limit of the spot size with respect to the desired crosstalk value. Each coefficient in equation (20) (hereinafter also referred to as “lower limit coefficient”) is obtained by fitting the average of these curves.

Is obtained.

FIG. 8 shows that in a wavelength multiplexed signal multiplexed at a frequency interval B ₀ , the frequency region (hereinafter referred to as “channel band”) of the width B ₀ of each signal frequency grid is determined by diffraction by a diffraction grating and by a condenser lens. Normalization band B of normalized spot size w / X that gives a crosstalk of −40 dB or less when the number of phase imparting elements included in the width reproduced on the spatial phase modulator by focusing is N = 100 It is a figure which shows / _B0 dependence. Here, B is a band below the allowable crosstalk value, and “signal frequency grid” is used in the meaning used in TTC standard JT-G694.1. However, the “signal frequency grid” does not have to completely coincide with that shown in TTC standard JT-G694.1, and the grids may have unequal frequency intervals. FIG. 9 shows the w / X dependence of the normalized band B / B ₀ of the normalized spot size w / X with an allowable crosstalk value of −40 dB or less for the number N of phase applying elements N = 20, 50, 100, 200. FIG. In both figures, when B / B ₀ increases, the graph is interrupted, which means that there is no spot size for realizing each band at each crosstalk value and N. The upper limit of the normalized spot size for securing the desired band (normalized band) varies depending on the desired band B and varies depending on the allowable crosstalk value and the number of phase imparting elements in the channel band. From FIG. 8, it can be seen that the normalized spot size has a substantially quadratic function dependence on the normalized band, and the coefficient varies depending on the allowable crosstalk value. Further, FIG. 9 shows that when the normalized band is constant, the normalized spot size increases almost in proportion to the channel band. Therefore, the normalized spot size can be expressed by the following formula.

  Here, m1 (XT, N), m2 (XT, N), and m3 (XT, N) are coefficients that change with respect to the allowable crosstalk value and the number of phase imparting elements (hereinafter also referred to as “upper limit coefficients”). It is.

A w / X-B / B ₀ curve is obtained for a plurality of allowable crosstalk values XT and the number N of phase imparting elements, and coefficients m1 (XT, N) and m2 (XT, N) of Expression (22) are obtained from the fitting coefficients. And m3 (XT, N) can be obtained. Specifically, when the w / X−B / B ₀ curve is obtained for the crosstalk value XT = −30, −40, −50, −60 dB, and the number of phase applying elements N = 20, 50, 100, 200, m1 (XT, N), m2 (XT, N) and m3 (XT, N) can each be simplified to a formula that depends only on the allowable crosstalk value XT, and m1 (XT , N), m2 (XT, N) and m3 (XT, N) can be well represented by the exponential dependence on the allowable crosstalk value XT,

Is obtained.

  From the above, the lower limit and upper limit of the spot size necessary to secure a desired band are determined by the equations (20) to (23).

(First embodiment)
FIG. 10 is a schematic view showing a first embodiment of the wavelength selective switch of the present invention. Light output from the input / output fiber 1 is reflected by the mirror 2, passes through the lens 3, and enters the diffraction grating 4. The light incident on the diffraction grating 4 is reflected at an emission angle corresponding to the wavelength, passes through the lens 3 again, is reflected by the mirror 2, and enters the spatial phase modulator 5. The light incident on the spatial phase modulator 5 is diffracted in a direction depending on the reproduced phase shift function, and again passes through the mirror 2, the lens 3, and the diffraction grating 4 in the desired input / output fiber array 1. To the fiber. As for the number of input fibers (input ports), the number of output fibers (output ports) is nine. The array of input / output fibers is in one row. The diffraction grating, the mirror, and the lens are adjusted so that the signal light in the wavelength region 1524.11 nm to 1570.42 nm can be switched. In this embodiment, LCOS (Liquid Crystal On Silicon) is used as a spatial phase modulator, and wavelength selective switches for a plurality of LCOS having different phase-giving element (pixel) width X and the number N of pixels of one signal light. The effect of the present invention was confirmed using

In the wavelength selective switch of the present embodiment, the positional relationship between the incident signal light and the diffracted signal light and the LCOS that is the spatial phase modulator is one of two orthogonal directions of the LCOS pixel plane as shown in FIG. The direction (hereinafter also referred to as “first direction”) is arranged so as to coincide with the wavelength dispersion direction of the input signal light incident on the LCOS, and the other direction (hereinafter also referred to as “second direction”). Only is the diffraction direction of the diffracted signal light. That is, in FIG. 11, the diffraction direction of the signal light passes on the line indicated by the dotted line. In this case, the range in which the frequency region B ₀ of the one-wavelength signal light of the wavelength multiplexed signal is reproduced on the spatial phase modulator is irrelevant to the linear dispersion, and therefore the upper limit of the spot size determined by the band limitation is not limited. become. On the other hand, the lower limit of the spot size is due to the fact that the phase modulation function is too coarse relative to the spot size, and the Gaussian beam focused on the spatial phase modulator feels a modulation function having a high frequency component. Since this occurs regardless of the line dispersion, the present embodiment is also applied. Therefore, the range of the beam spot size of the diffracted signal light on the LCOS in the wavelength selective switch of the present embodiment is not less than a value satisfying Expression (20).

  FIGS. 13, 14 and 15 show the crosstalk values of wavelength selective switches using LCOS with pixel widths of 2, 5 and 10 μm, respectively. One signal light has 100 pixels. In these figures, ◯, □, and Δ indicate crosstalk values when signal light input from the input port is output to the output ports 1, 5, and 9, respectively. The signal light wavelength is 1550.12 nm. From these figures, w / X = 0.68 provides an allowable crosstalk value of −40 dB (the allowable crosstalk of the wavelength selective switch is typically −40 dB or less as described in Non-Patent Document 2). Therefore, it can be seen that the boundary value is consistent with the result shown in FIG.

  FIGS. 16 and 17 show the wavelength at which the diffraction grating, mirror, and lens are selected and arranged so that the number of pixels of one signal light is N = 10 and 1000, respectively, when LCOS with a pixel width of 5 μm is used. The crosstalk of the selection switch is shown. In these figures, as in FIGS. 13 to 15, the ○ mark, □ mark, and Δ mark indicate the crosstalk values when the signal light input from the input port is output to the output ports 1, 5, and 9, respectively. The signal light wavelength is 1550.12 nm. 16, 17, and 14, it is understood that w / X = 0.68 is a boundary value for obtaining an allowable crosstalk value −40 dB even when the number of pixels of one signal light is different.

  In FIGS. 13 to 17, only the crosstalk at the time of output to the output ports 1, 5, and 9 is drawn with respect to the signal light having a wavelength of 15550.12 nm because the diagrams are difficult to see, but signals of other wavelengths are drawn. Similarly, when signal light is output to light or other output ports, the crosstalk value is −40 dB or more when w / X = 0.67, and −40 dB or less when w / X = 0.68. It has been confirmed that X = 0.68 is a boundary value for obtaining an allowable crosstalk value of −40 dB.

  The distance between the mirror, the lens, the diffraction grating, and the spatial phase modulator depends on the difference in the spread of the spatial spread (spot size) of the signal light electric field emitted from the fiber depending on the propagation distance (this is determined by the aperture ratio of the fiber. The optical means, the spectroscopic means, and the spatial phase modulator need only be installed so as to be within the range of the beam spot size defined in the present invention. Therefore, the configuration of the wavelength selective switch of the present embodiment shown in FIG. 10 shows an example of the wavelength selective switch of the present invention, and one direction orthogonal to the phase-giving element surface of the spatial phase modulator is the wavelength. If the other direction is the diffractive direction, the arrangement of mirrors, lenses and diffraction gratings is not limited to this, and optical elements not shown in this figure may be arranged as necessary. The effect of the present invention is not affected.

(Second embodiment)
The configuration of the wavelength selective switch of the second embodiment is almost the same as that of the first embodiment, but the number of input fibers (input ports) is 43, and the number of output fibers (output ports) is 43. Since the number of input / output ports) is large, the array of input / output fibers is in multiple rows. Therefore, in the second embodiment, the positional relationship between the incident signal light and the diffracted signal light and the LCOS that is the spatial phase modulator is the first of the two orthogonal directions of the pixel plane of the LCOS as shown in FIG. Are arranged so as to coincide with the wavelength dispersion direction of the input signal light incident on the LCOS, and this direction and the other second direction are the diffraction directions of the diffracted signal light. That is, in FIG. 12, the diffraction direction of the signal light is within the plane indicated by the dotted line. In this case, since the channel band that is a band occupied by one wavelength signal light of the wavelength multiplexed signal on the LCOS is related to the linear dispersion, an upper limit is defined for the spot size determined from the band limitation. Becomes a value satisfying the equation (22). In addition, the lower limit of the spot size is a value that satisfies Expression (20) as in the wavelength selective switch of the first embodiment.

  18, 19 and 20 show the crosstalk values of wavelength selective switches using LCOS with pixel widths of 2, 5, and 10 μm, respectively. One signal light has 100 pixels. In these figures, ◯, □, and △ indicate crosstalk values when signal light input from the input port is output to the output ports 1, 22, and 43, respectively. The signal light wavelength is 1550.12 nm. From these figures, similarly to the first embodiment, w / X = 0.68 is a boundary value for obtaining an allowable crosstalk value of −40 dB, and is consistent with the result shown in FIG. I understand.

  FIGS. 21 and 22 show the wavelength when a diffraction grating, a mirror, and a lens are selected and arranged so that the number of pixels of one signal light is N = 10 and 1000, respectively, when LCOS with a pixel width of 5 μm is used. The crosstalk of the selection switch is shown. As in FIGS. 18 to 20, these figures also indicate the crosstalk values when the signal light input from the input port is output to the output ports 1, 22, and 43, respectively. The signal light wavelength is 1550.12 nm. 21, 22, and 19, it can be seen that even when the number of pixels of one signal light is different, w / X = 0.68 is a boundary value for obtaining an allowable crosstalk value −40 dB.

  18 to 22, for the signal light having a wavelength of 15550.12 nm, only the crosstalk at the time of output to the output ports 1, 22, and 43 is drawn for the sake of difficulty in viewing the diagrams. Similarly, when signal light or signal light is output to other output ports, the crosstalk value is −40 dB or more when w / X = 0.67, and −40 B or less when w / X = 0.68. It can be confirmed that /X=0.68 is a boundary value for obtaining an allowable crosstalk value of −40 dB, as in the first embodiment.

  FIG. 23, FIG. 24, and FIG. 25 show the upper limit of the spot size obtained from the equations (22) and (23) and the band of the wavelength selective switch of this embodiment. In these drawings, the solid line is a curve obtained from the equations (22) and (23), and the two dotted lines are w / X curves of + 5% and −5% of the solid line, respectively. Further, the black circle mark, the black square mark, and the black triangle mark indicate the bandwidth of the wavelength selective switch when using LCOS having pixel widths of 2, 5, and 10 μm, respectively. In each wavelength selective switch, w / X is 10, The positions of the selected lenses and mirrors are adjusted so that the signal light beam is focused on the LCOS with a spot size of 5, 1.5. The number of pixels of one signal light is 100, the signal light wavelength is 1550.12 nm, and the output to the output port 1 is shown. Each point of the band with respect to the normalized spot size of the wavelength selective switch of the present embodiment is within ± 5% of the curve obtained by fixing N in Expression (22) and Expression (23), and Expression (22) and Expression It has been confirmed that (23) gives a spot size upper limit value for securing a desired band.

  FIGS. 26 to 28 and FIGS. 29 to 31 show bands of wavelength selective switches in which diffraction gratings, mirrors, and lenses are selected and arranged so that the number of pixels of one signal light is 10 and 1000, respectively. In these drawings, similarly to FIGS. 23 to 25, the signal light wavelength is 1550.12 nm, and the output to the output port No. 1 is shown. The normalized spot size of the wavelength selective switch from the combination of FIG. 23, FIG. 26, FIG. 29, the combination of FIG. 24, FIG. 27, FIG. 30, and the combination of FIG. It has been confirmed that it is proportional to the number of pixels. Therefore, when FIGS. 23 to 31 are combined, each point of the band with respect to the normalized spot size of the wavelength selective switch of the present embodiment is within ± 5% of the curve obtained from the equations (22) and (23). It has been confirmed that (22) and Equation (23) give a spot size upper limit value for securing a desired band.

  23 to 31, only the crosstalk at the time of output to the output port 1 is drawn with respect to the signal light having a wavelength of 1550.12 nm in order to make it difficult to see the drawings. Similarly, it is confirmed that the band is within the range of ± 5% of the curves obtained from the equations (22) and (23) when the signal light is output to the output port of the above.

1 Input / output fiber 2 Mirror 3 Lens 4 Diffraction grating 5 Spatial phase modulator

Claims (7)

Spectroscopic means for demultiplexing wavelength-multiplexed signal light in which signal light of a plurality of wavelengths is wavelength-multiplexed for each wavelength;
A spatial phase modulator having a plurality of phase imparting elements pixelated with a width X that provides a phase shift to the light;
A wavelength selective switch comprising optical means for focusing the signal light for each wavelength demultiplexed by the spectroscopic means on the spatial phase modulator,
In the first and second directions orthogonal to each other of the phase applying element surface of the spatial phase modulator, the first direction is a chromatic dispersion direction, and the second direction is a diffraction direction,
The spatial spread of the electric field of the signal light collected on the spatial phase modulator is defined by the spot size w of the Gaussian beam,
The wavelength selective switch, wherein the range of the spot size w is limited by the following equation.

Here, n1 (XT, N), n2 (XT, N) and n3 (XT, N) are the first and second values depending on the desired crosstalk value XT and the number N of phase imparting elements in the channel band, respectively. And the third lower limit coefficient. The first to third lower limit coefficients are:

The wavelength selective switch according to claim 1, wherein:   The wavelength selective switch according to claim 1, wherein w / X ≧ 0.68 is satisfied. The first direction is a chromatic dispersion direction and a diffraction direction, and the second direction is a diffraction direction,
2. The wavelength selective switch according to claim 1, wherein a range of the spot size w is limited by the following expression.

Here, B ₀ is a frequency interval B of the wavelength multiplexed signal light, and a band in which the dispersion compensation value falls within an allowable range from a desired value, m 1 (XT, N), m 2 (XT, N) and m 3 (XT, N ) Are first, second, and third upper limit coefficients that depend on the desired crosstalk value XT and the number N of phase imparting elements in the channel band, respectively. The first to third upper limit coefficients each depend only on the crosstalk value XT,
5. The wavelength selective switch according to claim 4, wherein the first to third upper limit coefficients are coefficients that depend on an exponent with respect to the crosstalk value XT. The first to third lower limit coefficients are:

And
6. The first to third upper limit coefficients are represented by the following formulas using a coefficient having an error of ± 5% with respect to a desired crosstalk value XT. Wavelength selective switch.

  The wavelength selective switch according to claim 6, wherein w / X ≧ 0.68 is satisfied.

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